Risk assessment of highly active human pharmaceuticals in the

Risk assessment of highly active human pharmaceuticals in the

Umweltforschungsplan
des Bundesministeriums für Umwelt,
Naturschutz und Reaktorsicherheit
Förderkennzeichen (UFOPLAN) 370765400
Risk assessment of highly active human pharmaceuticals
in the environment: Identification, development and
application of a new concept for their identification
von
Dr. Verena Christen & Prof. Dr. Karl Fent
Fachhochschule Nordwestschweiz
Hochschule für Life Sciences
Gründenstrasse 40
CH-4132 Muttenz
IM AUFTRAG
DES UMWELTBUNDESAMTES
August 2009
1
Berichts-Kennblatt
2.
3.
1.
Berichtsnummer
4.
Titel des Berichts
Risikobewertung von hochaktiven humanen Medikamenten in der Umwelt: Identifikation, Entwicklung und
Anwendung eines neuen Konzeptes zur Identifikation solcher hoch aktiven Substanzen
Autor(en), Name(n), Vorname(n)
8.
Abschlussdatum
Fent Karl
August 2009
Christen Verena
9.
Veröffentlichungsdatum
Durchführende Institution (Name, Anschrift)
Oktober 2009
5.
6.
Fachhochschule Nordwestschweiz
Hochschule für Life Sciences
Gründenstrasse 40
CH-4132 Muttenz
7.
10.
UFOPLAN-Nr.
UBA-FB FKZ 3707 65 400
11.
Seitenzahl
94
12.
Literaturangaben
132
13.
Tabellen und Diagramme
15
14.
Abbildungen
8
Fördernde Institution (Name, Anschrift)
Umweltbundesamt, Postfach 14 06, 06813 Dessau-Roßlau
15.
Zusätzliche Angaben
16. Kurzfassung
In den letzten Jahren wurden dank verbesserter Analysemethoden eine Vielzahl von Medikamenten im Abwasser, im
Oberflächenwasser und Grundwasser im Konzentrationsbereich von Nanogramm bis Mikrogramm pro Liter
nachgewiesen. Mögliche Effekte dieser Stoffe auf die Umwelt sind bisher aber noch kaum bekannt.
Seit 2006 ist die neue Richtlinie der European Medicines Agency (EMEA) zur Umweltbeurteilung von neuen ArzneiMitteln in Kraft. Danach wird ihr Umweltrisiko in einem zweistufigen Verfahren bestimmt. In der ersten Stufe wird die
potentielle Umweltkonzentration eines Medikamentes abgeschätzt. Liegt diese unter 0.01 Mikrogramm pro Liter, wird
wird das Medikament als unbedenklich für die Umwelt eingestuft. Wenn die Konzentration den Schwellenwert von
10 Nanogramm pro Liter überschreitet, sind experimentelle Studien zum Verbleib und zur Wirkung des Arzneimittels
In der Umwelt durchzuführen. Da es jedoch Medikamente wie Ethinylestradiol und Levonorgestrel gibt, die Effekte
auf aquatische Organismen unterhalb des Schwellenwertes ausüben, müssen diese identifiziert werden. Bisher gibt es
Aber keine Kriterien, anhand derer solche Substanzen identifiziert werden können.
Ziel dieses Vorhaben ist es, solche hochaktiven Substanzen anhand vorhandener Daten und der wissenschaftlichen
Literatur zu identifizieren. Des Weiteren wird ein neues Konzept erarbeitet, anhand dessen hochaktive Substanzen
identifiziert werden können. Dabei werden Kriterien definiert. Das neue Konzept basiert auf dem Wirkmechanismus
der Substanz. Zudem wird der Grad der Homologie zwischen dem Zielrezeptor beim Menschen und dem potentiellen
Rezeptor bei Organismen in der Umwelt betrachtet. Ein weiteres Kriterium ist die Bedeutung des durch den beteiligten
Rezeptor regulierten Signalwegs in der Zelle und des Stoffwechselwegs. Dabei werden wichtige biologische und physio­
logische Prozesse definiert. Zudem werden toxikologische Daten mit berücksichtigt. Das neue Konzept wird anhand von
Beispielen verifiziert. Schliesslich wird das neue Konzept mit Modellen wie dem Fisch-Plasma-Modell und
einem QSAR-Modell verglichen. Die Analyse zeigt, dass mit dem neuen Konzept hochaktive Substanzen erkannt werden
können.
17. Schlagwörter
EMEA Guideline. Umweltrisikobeurteilung von Arzneimitteln. Hochaktive Arzneimittel in der Umwelt. Konzept zur
Identifikation von hochaktiven Substanzen. Fisch-Plasma-Modell. QSAR-Modell.
18.
Preis
83 000 EUR
19.
20.
2
Report Cover Sheet
1.
4.
Report No.
2.
3.
UBA-FB
Report Title
Risk assessment of highly active human pharmaceuticals in the environment: Identification, development and application
Of a new concept for their identification
5.
Autor(s), Family Name(s), First Name(s)
Fent Karl
Christen Verena
6.
Performing Organisation (Name, Address)
University of Applied Sciences Northwestern Switzerland
School of Life Sciences
Gründenstrasse 40
4132 Muttenz - Switzerland
7.
8.
Report Date
August 2009
9.
Publication Date
October 2009
10.
UFOPLAN-Ref. No.
UBA-FB FKZ 3707 65 400
11.
No. of Pages
94
12.
No. of Reference
132
13.
No. of Tables, Diagrams
15
14.
No. of Figures
8
Sponsoring Agency (Name, Address)
Umweltbundesamt, Postfach 14 06, 06813 Dessau-Roßlau
15.
Supplementary Notes
16.
Abstract
Due to more sensitive analytical methods, an increasing number of pharmaceuticals are detected in wastewater,
surface and ground water at concentrations from nanogram to microgram per litre in the last decade. Possible
effects of these compounds in the environment are only marginally unknown. The guideline of the European
Medicines Agency (EMEA) for the environmental risk assessment of new pharmaceuticals is in force since 2006.
It consists of two steps. In the first step, the potential environmental concentration of the pharmaceutical is
estimated. If the concentration is below 10 nanogram per litre, the pharmaceutical is classified as harmless. If
the concentration is above this threshold, studies about the fate and effects of the new drug are necessary.
However, drugs such as ethinylestradiol and levonorgestrel exist that exhibit effects to aquatic organisms below the
threshold. So far, there are no criteria defined, nor concepts to identify such highly active compounds.
The goal of this project is to identify such highly active compound by use of existing data banks and the scientific
literature. A concept for the identification of highly active compounds is developed and established. The new
concept is based on the mode of action of a compound. In addition, the homology between the human drug target
and the potential targets in organism in the environment is taken into account. Another criteria is the impact
on the regulated cellular signalling pathways affected by the drug. Consequently, critical biological and physiological
processes are defined. Furthermore, toxicological data are considered. The new concept is applied to some
pharmaceuticals for verification. Finally the new concept is compared to existing models such as the fish plasma
model and a QSAR model. The analysis shows that the new model, highly active compounds can be
identified. Future applications will demonstrate its usefulness within the regulatory context.
17.
Keywords
EMEA guideline. Environmental risk assessment of pharmaceuticals. Highly active compounds in the
Environment. Concept for the identification of highly active compounds. Fish plasma model. QSAR model
18.
Price
83 000 Euro
19.
20.
3
Table of contents
Summary
4
1. Introduction
5
1.2 Pharmaceuticals in the environment
5
1.3 Ecotoxicological effects
7
1.4 Environmental risk assessment of pharmaceuticals
according to the European Medicines Agency (EMEA)
8
1.5 Drug Categorization
9
1.5.1 Consumption of pharmaceuticals in Germany
9
1.5.2 Main groups of pharmaceuticals
10
2. Identification of highly active pharmaceuticals
30
2.1 Identification of existing “however compounds”
30
2.2 Identification of important pathways of “HC”
36
3. Concepts for identification of “HC”
48
3.1 Mode of action concept
48
3.1.1 Consideration of toxicological data
48
3.1.2 Description of the mode of action concept
52
3.1.3 Classification of regulated effects through conserved
receptors
56
3.1.4 Application of the mode of action concept for selected
pharmaceuticals
59
3.2 The fish plasma model concept – an additional/complementary
model to identify “HC”
61
3.2.1 Description of the model
62
3.2.2 Application for selected pharmaceuticals
63
3.2.3 Comparison of fish plasma model and mode of action concept
66
3.4 The QSAR concept
67
3.4.1 Quantitative Structure-Activity Relationships QSAR
67
3.4.2 Analysis of binding affinities of chemicals or pharmaceuticals
to specific human receptors using VirtualToxLab
68
3.4.3 Dimensionality of QSAR approaches
69
3.4.4 ECOSAR
72
3.5 Comparison of fish plasma model, mode of action concept
4
and VirtualTox lab
73
3.6 Conclusions
74
4. Environmental risk assessment: Phase II: environmental
fate and effects analysis
75
5. References
80
Appendix
90
5
Summary
Over the last decade, traces of pharmaceuticals have been detected in many countries at
concentrations in the ng/L to µg/L range in wastewater treatment plant effluents, surface
water, seawater and groundwater. The effects of these compounds on the environment are
not well investigated so far. Most effects are shown in acute toxicity experiments, but chronic
toxicity data are missing for most of the commonly used pharmaceuticals. To estimate the
potential environmental risk of human medicinal drugs, the European Medicines Agency
(EMEA) issued in 2006 the “guideline on the environmental risk assessment of medicinal
products for human use”. According to this guideline the potential risk of pharmaceuticals to
the environment is estimated by a step-wise procedure. During phase I the predicted
environmental concentration (PEC) of a pharmaceutical is assessed in the surface water. If
the PEC value is below 0.01 µg/L and no other environmental concerns are apparent, it is
assumed that the medicinal product is unlikely to represent a risk for the environment. If the
PEC value is equal or above 0.01 µg/L a phase II environmental fate and effect analysis is
performed.
The threshold of 0.01 µg/L may be too high for very specific and highly potent
pharmaceuticals like hormones, which can have effects on the environment at concentrations
below 0.01 µg/L. Some compounds such as 17-α-ethinylestradiol and levonorgestrel display
effects at aquatic organisms at concentrations below 10 ng/L. The goal of the project was the
identification of such highly potent and highly selective pharmaceuticals and pharmaceuticals
groups, which could exhibit a potential effect on the environment at concentrations below the
threshold value of 0.01 µg/L. To identify these highly active compound (“HC”), various
databases and the scientific literature were used. The ultimate aim of the present project was
the development of a concept for the identifcation of such “HC”. We developed a concept
based on the mode of action of pharmaceuticals to identify compounds with potential
ecotoxicological risk at low concentrations. This concept is based on (1) the mode of action
of the pharmaceutical taking into consideration of toxicological data, (2) the degree of
sequence homology between the human drug target and the potential target in aquatic
organisms and (3) on the importance of the pathways affected by the drug. We evaluated the
usefulness of this concept to other potential approaches such as the fish plasma model
(Huggett et al., 2003) and a QSAR model (VirtualTox Lab). In the fish plasma model the fish
plasma concentration of drugs is estimated and compared with the human therapeutic
plasma concentration for predicting the potential risk. The QSAR (VirtualTox lab) model is
based on the binding affinity of pharmaceuticals to human receptors for risk assessment. All
concepts were evaluated using a set of seven pharmaceuticals. Application of the three
concepts to the selected pharmaceuticals resulted in most, but not all, cases the same
6
categorization. The mode of action concept was most promising for identification of
compounds of high ecotoxicological risks. Furthermore, the inclusion of toxicological data in
the risk assessment provided useful hints in terms of identification of mode of actions of the
compounds and its potential potential ecotoxicological risk at low concentrations.
1. Introduction
1.2 Pharmaceuticals in the environment
Pharmaceuticals are a class of emerging environmental contaminants that are extensively
and increasingly being used in human and veterinary medicine. The consumption of
pharmaceuticals is substantial. In the European Union (EU) about 3000 different substances
are used in human medicine. Also a large number of pharmaceuticals are used in veterinary
medicine. The main categories of human pharmaceuticals and the most commonly used
products are anti-inflammatory drugs/analgesics, antibiotics, lipid regulators, beta blockers,
steroids and related hormones (Daughton et al., 1999; Halling-Sorensen et al., 1998). For
example, in England, Germany, the United States, Denmark and Australia, the amounts for
the most frequently used drugs are in the hundreds of tons per year (Huschek et al., 2004;
Jones et al., 2002).
These compounds are designed to have a specific mode of action, and many show long half­
lives in the body. Pharmaceuticals usually undergo transformations in the human body,
resulting in the release of significant amounts of metabolites. Pharmaceuticals and their
metabolites can enter the environment mainly via excretion and disposal in wastewater.
Municipal and hospital wastewaters are the most important sources of human
pharmaceutical compounds, with contributions also from wastewater, manufacturers and
landfill leachates, and from disposal of unused medicines into the environment. Application to
fields and subsequent runoff, and direct application in aquaculture, are the main sources of
veterinary pharmaceuticals in the aquatic environment (Fent et al., 2006; Johnson et al.,
2006; Kay et al., 2005). In wastewater treatment, two elimination processes are generally
important: adsorption to suspended solids (sewage sludge) and biodegradation. Adsorption
is dependent on both hydrophobic and electrostatic interactions of the pharmaceutical with
particles and microorganisms. Biodegradation can occur either in aerobic/anaerobic zones in
activated sludge treatment, or anaerobically in sewage sludge digestion. Because of an
incomplete elimination in wastewater treatment plants, pharmaceuticals and their metabolites
are found in surface waters. The elimination efficiencies of pharmaceuticals span a large
7
range. Figure 1 shows the elimination rates of some non-steroid-anti-inflammatory drugs
(NSAID) as an example.
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Fig.1: Removal efficiency of several NSAID (percentage) in different wastewater treatment
plants. Salicylic acid (1), diclofenac (2-6), ibuprofen (7,8), naproxen (9) and paracetamol.
Removal rates are variable, even for the same pharmaceutical between different treatment
plants (Fent et al., 2006).
As consequence of incomplete elimination in wastewater treatment plants (and where
treatment plants are lacking), pharmaceuticals and their metabolites enter the aquatic
environment. The occurrence of pharmaceuticals was first reported in 1976, clofibric acid
was detected in treated wastewater in the USA at concentrations from 0.8 to 2 µg/L (Fent et
al., 2006). In 1981 the occurrence of pharmaceutical compounds in river waters in the UK
was reported (Fent et al., 2006), and in 1986 ibuprofen and naproxen were detected in
wastewaters in Canada (Fent et al., 2006). In the meantime pharmaceuticals have been
detected in low concentrations in many countries in many environmental samples, for
example wastewater treatment plant effluents, surface water, seawater, and groundwater
(Daughton et al., 1999; Fent et al., 2006; Halling-Sorensen et al., 1998). The measured
concentrations in sewage treatment plant effluents are generally in the range of ng/L to µg/L
and in rivers, lakes and seawater in ng/L (Kolpin et al., 2002). In the last few years,
knowledge about environmental occurrence of pharmaceuticals has increased to a large
extent due to new analytical techniques able to detect polar compound at trace quantities.
Since a few years, research about the fate of pharmaceuticals in the environment is
increasing. Typical depletion processes in surface waters are: biodegradation in sediments,
phototransformation, hydrolysis and binding to sediments (Kolpin et al., 2002, HallingSorensen et al., 1998, 2003; Ingerslev et al., 2001). The low volatility of pharmaceutical
products indicates that distribution in the environment will occur primarily by aqueous
8
transport, but also by food-chain dispersal. Once in surface water, biotransformation may
occur, but abiotic transformation reactions are also important. Photodegradation can play an
important role at the water surface and hydrolysis may occur. There is no or very little
information about the bioaccumulation potential of pharmaceuticals in biota and food chains
with the exception of diclofenac, which accumulates in the prey of vultures (Oaks et al.,
2004).
1.3 Ecotoxicological effects
Pharmaceuticals are designed to target specific metabolic and molecular pathways in
humans and animals. When introduced into the environment they may affect the same
pathways in vertebrates and invertebrates having identical/similar targets. So far, very little is
known about possible counterparts of human target biomolecules of pharmaceuticals in
invertebrates. In addition, for many pharmaceuticals the specific mode of action is not well
characterized and often not only one but many different modes of action occur. Therefore
specific toxicity analysis in lower animals is difficult to perform. Pharmaceuticals are
assessed for their long-term toxicity by standard tests according to existing guidelines (for
example OECD) using laboratory organisms of different trophic levels such as algae,
zooplankton, other invertebrates and fish. Beyond laboratory investigations, some
mathematical models were developed to estimate or predict ecotoxicological effects of
chemicals. Beside the evaluation of acute effects of pharmaceuticals for aquatic animals, the
characterisation of their chronic potential is very important, because many aquatic species
are exposed over long periods of time or even for their entire life. But there is a general lack
of chronic data and the existing ones do often not investigate the important key targets
(which are based on the mode of actions) and they do not address the question in different
organisms. Investigations about chronic toxicity over different life stages have only rarely
been performed.
Life cycle analyses are not reported so far, except for the synthetic steroid 17α­
ethinylestradiol (EE2), contained in contraceptive pills. Oviparous females of fish, reptiles,
amphibians and birds secrete the steroid hormone 17β-estradiol from the thecal cells
surrounding the ovaries, which in turn migrates through the bloodstream to the liver to induce
vitellogenin synthesis. Activation of vitellogenin transcription occurs through transcriptional
activation once the estrogenic steroid binds to the nuclear estrogen receptor. Exposure of
male fish to EE2 leads to the activation of this pathway therefore vitellogenin transcription
can be used as a marker for estrogen exposure. EE2 shows estrogenic effects at extremely
9
low and environmentally relevant concentrations. The ecotoxicological effects of EE2 were
studied in a fish full life-cycle study using fathead minnows (Länge et al., 2001). EE2 had a
strong impact on several key health indices, including gross development and growth, gonad
development, sex determination and reproductive maturity. The effects of EE2 on
development in the early life stages of fathead minnows were most obvious. After 56 days
posthatch, the effect of EE2 on larval growth was severe and ovotestis was present with
LOEC (lowest-observed-effect concentration) of 4 ng/L and NOEC (no-observed-effect
concentration) of 1 ng/L. Fish exposed to 4 ng/L EE2 showed no male secondary sex
characteristics at any age. Overall for all the endpoints monitored during the EE2 full life­
cycle study, the biological derived NOEC was 1 ng/L (Lange et al., 2001). Induced
transcription of vitellogenin mRNA was observed in fathead minnows exposed to
concentrations as low as 2 ng/L EE2 for 24 h (Miracle et al., 2006)
1.4 Environmental risk assessment of pharmaceuticals according to
the European Medicines Agency (EMEA)
According to the guideline of EMEA (2006) the assessment of the potential risks to the
environment is a step-wise procedure, consisting of two phases. In the first phase the
exposure of the environment to the drug substance is estimated. By application of the
following mathematical formula the PEC of a pharmaceutical in the surface water is
calculated during the first phase.
Maximum daily dose consumed per inhabitant * percentage of market penetration (Fpen)
Amount of wastewater per inhabitant per day * dilution factor
The default value for Fpen (1 %) should be used unless the Fpen can be refined based on
published epidemiological data.
If the PECsurfacewater value is below 0.01 µg/L and no other environmental concerns are
apparent, it is assumed that the medicinal product is unlikely to represent a risk for the
environment. If the PECsurfacewater value is equal or above 0.01 µg/L, a Phase II environmental
fate and effect analysis has to be performed. During this Phase II, the fate of the substance
in the sewage treatment plant is investigated by performing ready biodegradability test, and
analyzing the sorption behaviour of the substance to sewage sludge and soil, the distribution
between octanol and water, and the transformation in water-sediment-systems. Depending on
the outcome of the fate tests, further testing in Tier B might be necessary, for example on fate
10
and effects in soil or on bioaccumulation. The effect on sewage treatment plants is tested in a
respiration inhibition test. Standard long-term toxicity tests on fish, Daphnia and algae are
performed to predict the no-effect concentration (PNECwater). The PNECwater is used to
calculate the ratio PECsurface
water
versus PNECwater. If this value is below 1, there is no further
ecotoxicological testing in the aquatic environment needed. The EMEA guideline
acknowledges that the threshold of 0.01 µg/L may not be applicable for very specific and
highly potent pharmaceuticals like hormones, which can have effects on the environment at
concentrations below 0.01µg/L. These compounds enter Phase II irrespective of the predicted
environmental concentration. In Phase II, a tailored risk assessment taking into account the
mode of action is required. Therefore the pharmaceutical industry has to evaluate for each
product, if it is known or if it could be expected that the effect concentration is below
0.01µg/L. Pharmaceuticals that fulfil this criteria are so called compounds of high activity
“HC”.
1.5 Drug Categorization
To get an overview of the mostly used pharmaceuticals prescribed in Germany and of the
used amount, we did an intensive database search to make a categorization of the most
prescribed pharmaceuticals.
1.5.1 Consumption of pharmaceuticals in Germany
Pharmaceuticals are a class of emerging environmental contaminants that are extensively
and increasingly being used in human and veterinary medicine. The consumption of
pharmaceuticals is substantial. In Germany about 3000 different substances are used in
human medicine (Arzneiverordnungs-Report 2007). Also a large number of pharmaceuticals
are used in veterinary medicine. Figures 2 and 3 show the consumption of pharmaceuticals
in Germany 2006.
11
Fig. 2: Consumption of pharmaceuticals in Germany 2006 (prescriptions in millions,
Arzneiverordnungs-Report 2007, Springer- Verlag Berlin-Heidelberg 2008): 1, drugs to treat
heart disorders (109.2), 2, antibiotics (37.3), 3, drugs to treat inflammation and rheumatism
(34.5), 4, pain killers (31.9), 5, anti diabetic drugs (26.9), 6, psycholeptics (25.9), 7, drugs to
treat asthma (24.7), 8, drugs to treat ulcers (21.2), 9, stimulant drugs (18.5), 10, drugs to
treat thyroid gland disorders (18.2) and 11, blood lipid lowering agents (14.1)
Fig. 3: Prescribed drugs to treat heart disorders in millions prescriptions 2006 in Germany
(Arzneiverordnungs-Report 2007, Springer- Verlag Berlin-Heidelberg 2008): 1: angiotensin
receptor inhibitors (40.0), 2: beta blockers (32.7), 3: diuretics (19.9) and 4: calcium channel
inhibitors (16.6)
1.5.2 Main groups of pharmaceuticals
In order to have a general category to which the many different pharmaceuticals can be
assigned to, we have followed a common scheme according to the working mechanism of
the drug. To make this general categorization of pharmaceuticals, the following sources were
used: Allgemeine und spezielle Pharmakologie und Toxikologie, Urban und Fischer, 9.
Auflage
2004;
www.meds.com;
www.drugs.com;
www.mayoclinic.com/health/drug­
information). In the following, the different groups are briefly listed.
12
Antihypertensives - drugs to treat heart disorders
Antihypertensives are a class of drugs that are used in medicine and pharmacology to treat
hypertension (high blood pressure). There are many classes of antihypertensives, which - by
varying means - act by lowering blood pressure. A reduction of blood pressure by 5-6 mmHg
can decrease the risk of stroke by 40% and of coronary heart disease by 15-20%. The
likelihood of dementia, heart failure, and mortality from cardiovascular disease is also
reduced.
Angiotensin-converting enzyme inhibitors
Inhibitors of angiotensin-converting enzyme (ACE inhibitors), are a group of pharmaceuticals
that are used primarily in the treatment of hypertension and congestive heart failure. ACE
inhibitors lower arteriolar resistance and increase venous capacity. Beside of these effects,
ACE inhibitors increase cardiac output and cardiac index, stroke work and volume. Other
important functions of this group of drugs are the lowering of renovascular resistance and the
increasing of the excretion of sodium in the urine.
Angiotensin II receptor antagonists
Their main use is to treat hypertension (high blood pressure), diabetic nephropathy (kidney
damage due to diabetes) and congestive heart failure. Angiotensin II receptor antagonists
are primarily used for the treatment of hypertension, where the patient is intolerant of ACE
inhibitor therapy.
Aldosterone antagonists
Aldosterone is a steroid hormone produced by the outer-section of the adrenal cortex in the
adrenal gland, and acts on the distal tubules and collecting ducts of the kidney to cause the
conservation of sodium, secretion of potassium, increased water retention, and increased
blood pressure. The overall effect of aldosterone is to increase reabsorption of ions and
water in the kidney. Aldosterone antagonist refers to drugs which antagonise the action of
aldosterone. This group of drugs is often used as adjunctive therapy, in combination with
other drugs for the management of chronic heart failure.
13
Adrenergic receptor antagonists - beta blockers
Beta blockers (β-blockers) are a class of drugs used for the management of hypertension,
cardiac arrhythmias and cardioprotection after myocardial infarction. Beta blockers block the
action of β-adrenergic receptors, associated with the sympathetic nervous system. There are
three known types of beta receptors, designated β1, β2 and β3. β1-adrenergic receptors are
located mainly in the heart and in the kidneys. β2-adrenergic receptors are located mainly in
the lungs, gastrointestinal tract, liver, uterus, vascular smooth muscle, and skeletal muscle.
β3-receptors are located in fat cells. Stimulation of β1 receptors induces a positive effect on
the heart and increases cardiac conduction velocity and automaticity. Stimulation of β1
receptors on the kidney causes renin release. Stimulation of β2 receptors induces smooth
muscle relaxation (resulting in vasodilation and bronchodilation amongst other actions),
induces tremor in skeletal muscle, and increases glycogenolysis in the liver and skeletal
muscle. Stimulation of β3 receptors induces lipolysis. Beta blockers inhibit these normal
sympathetic actions, but have minimal effect on resting subjects. That is, they reduce the
effect of excitement/physical exertion on heart rate and force of contraction, dilation of blood
vessels and opening of bronchi, and also reduce tremor and breakdown of glycogen. There
exist non-selective beta blockers like propanolol, beta-1 selective blockers like atenolol and
beta-2 selective agents.
Adrenergic receptor agonist
The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that
are targets of the catecholamines. Adrenergic receptors specifically bind their endogenous
ligands, the catecholamines adrenaline and noradrenaline (called epinephrine and
norepinephrine in the United States), which leads to their activation. Many cells possess
these receptors, and the binding of an agonist will generally cause a sympathetic response
(i.e. the fight-or-flight response). For instance, the heart rate will increase and the pupils will
dilate, energy will be mobilized, and blood flow diverted from other, non-essential, organs to
skeletal muscle.
Diuretics
A diuretic is any drug that elevates the rate of urination (diuresis). There are several
categories of diuretics. All diuretics increase the excretion of water from the body, although
each class of diuretic does so in a distinct way. In medicine, diuretics are used to treat heart
failure, liver cirrhosis, hypertension and certain kidney diseases.
14
The antihypertensive
actions of some diuretics (thiazides and loop diuretics in particular) are independent of their
diuretic effect. That is, the reduction in blood pressure is not due to decreased blood volume
resulting from increased urine production, but occurs through other mechanisms and at lower
doses than that required to produce diuresis. Indapamide was specifically designed with this
in mind, and has a larger therapeutic window for hypertension (without pronounced diuresis)
than most other diuretics. Chemically, diuretics are a diverse group of compounds that either
stimulate or inhibit various hormones that naturally occur in the body to regulate urine
production by the kidneys.
Calcium channel blockers
Calcium channel blockers are a class of drugs and natural substances with effects on many
excitable cells of the body, like the muscle of the heart, smooth muscles of the vessels or
neurons. The main action of calcium channel blockers is to decrease the blood pressure. It is
for this action that it is used in individuals with hypertension.
Vasolidators
A vasodilator is a drug or chemical that relaxes the smooth muscle in blood vessels, which
causes them to dilate. Dilation of arterial blood vessels (mainly arterioles) leads to a
decrease in blood pressure.
Antibiotics
An antibiotic is a chemotherapeutic agent that inhibits or abolishes the growth of micro­
organisms, such as bacteria. The term originally referred to any agent with biological activity
against living organisms; however, "antibiotic" now refers to substances with anti-bacterial,
anti-fungal, or anti-parasitical activity. The first widely used antibiotic compounds used in
modern medicine were produced and isolated from living organisms, such as the penicillin
class produced by fungi of the genus Penicillium, or streptomycin from bacteria of the genus
Streptomyces. At the highest level, antibiotics can be classified as either bactericidal or
bacteriostatic. Bactericidals kill bacteria directly where bacteriostatics prevent them from
dividing. Antibiotics are only intended to be used in human medicine by a prescription.
However, they are also used in other applications such as veterinary medicine, materials and
product conservation etc.
15
Analgesics
An analgesic (painkiller) is any member of the diverse group of drugs used to relieve pain
(achieve analgesia). The word analgesic derives from Greek an- ("without") and -algia
("pain"). Analgesic drugs act in various ways on the peripheral and central nervous systems;
they include paracetamol (acetaminophen), the non-steroidal anti-inflammatory drugs
(NSAIDs) such as the salicylates, narcotic drugs such as morphine, synthetic drugs with
narcotic properties such as tramadol, and various others.
Non-steroidal anti-inflammatory drugs (NSAID)
NSAIDs are drugs with analgesic, antipyretic and anti-inflammatory effects - they reduce
pain, fever and inflammation. The term "non-steroidal" is used to distinguish these drugs from
steroids, which (among a broad range of other effects) have a similar anti-inflammatory
action. The most prominent members of this group of drugs are aspirin, ibuprofen, and
naproxen partly because they are available over-the-counter in many countries. NSAIDs are
usually indicated for the treatment of acute or chronic conditions, where pain and
inflammation are present. The anti-inflammatory action of NSAID rests in their ability to inhibit
the activity of the cyclooxygenase (COX) enzyme, which in turn results in a diminished
synthesis of proinflammatory prostaglandins (Burian, 2005). Prostaglandins act (among
others) as messenger molecules in the process of inflammation. In the early 1990s, COX
was demonstrated to exist as two distinct isoforms (Fu et al., 1990). COX-1 is constitutively
expressed as a housekeeping enzyme in nearly all tissues, and mediates physiological
responses (e.g., cytoprotection of the stomach, platelet aggregation). COX-2 is expressed in
cells that are involved in inflammation (e.g., macrophages, monocytes, synoviocytes). It
represents the isoform that is primarily responsible for the synthesis of the prostanoids
involved in pathological processes, such as acute and chronic inflammatory states. Most
NSAIDs act as non-selective inhibitors of the COX-1 and COX-2. NSAIDs act at the COX
active site in several ways (Marnett et al., 1999). Aspirin irreversibly inactivates both COX-1
and COX-2 by a covalent modification of the active site. This modification interferes with the
binding of the substrate (arachidonic acid) to the enzyme. By contrast, reversible competitive
inhibitors of both isoforms (e.g., mefenamate, ibuprofen) compete with arachidonic acid for
the COX active site. A third class of NSAIDs (e.g., flurbiprofen, indomethacin) causes a slow,
time-dependent reversible inhibition of COX-1 and COX-2. This is caused by an induction of
a conformational change of the enzyme.
16
Opiates and morphinomimetics
Morphine, the archetypal opioid, and various other substances (e.g. codeine, oxycodone,
hydrocodone, diamorphine, pethidine) all exert a similar influence on the cerebral opioid
receptor system. Dosing of all opioids may be limited by opioid toxicity (confusion, respiratory
depression, myoclonic jerks and pinpoint pupils), but there is no dose ceiling in patients who
tolerate this.
Opioids, while very effective analgesics may have some unpleasant side-effects, like nausea
and vomiting. When used appropriately, opioids and similar narcotic analgesics are
otherwise safe and effective, however there is a risk of addiction and the adaption of the
body to the drug. Often the dose must be increased. In a chronic disease the no ceiling limit
of the drug comes into play. However, there is still a toxic dose even if the body has become
used to lower doses.
Statins
The statins (or HMG-CoA reductase inhibitors) form a class of hypolipidemic drugs used to
lower cholesterol levels in people with or at risk of cardiovascular disease. They lower
cholesterol by competitively inhibiting the enzyme HMG-CoA reductase, which is the rate­
limiting enzyme of the mevalonate pathway of cholesterol synthesis (Endo, 1992). Inhibition
of this enzyme in the liver stimulates LDL receptors, resulting in an increased clearance of
low-density lipoprotein (LDL, so-called "bad cholesterol") from the bloodstream and a
decrease in blood cholesterol levels (Ma et al., 1986). Most circulating cholesterol is
manufactured internally, in amounts of about 1000 mg/day, via steroid biosynthesis through
the HMG-CoA reductase pathway. Cholesterol, both from dietary intake and secreted into the
duodenum as bile from the liver, is typically absorbed at a rate of 50% by the small
intestines. Cholesterol is not water-soluble, and is therefore carried in the blood in the form of
lipoproteins. The relative balance between these lipoproteins is determined by various
factors, including genetics, diet, and insulin resistance. Low density lipoprotein (LDL) and
very low density lipoprotein (VLDL) carry cholesterol toward tissues, and elevated levels of
these lipoproteins are associated with atheroma formation (fat-containing deposits in the
arterial wall) and cardiovascular disease. High density lipoprotein, in contrast, carries
cholesterol back to the liver and is associated with protection against cardiovascular disease.
17
Fibrates
Fibrates are used for a range of metabolic disorders, mainly hypercholesterolemia (high
cholesterol), and are therefore hypolipidemic agents. Fibrates are used as monotherapy or in
combination therapy with statins. Although less effective in lowering LDL, fibrates improve
HDL and triglyceride levels (i.e. increase HDL levels and decrease triglyceride levels).
Although used clinically since at least the 1930s, the mechanism of action of fibrates
remained undiscovered until, in the 1990s, it was discovered that fibrates activate
peroxisome proliferator-activated receptors (PPAR), especially PPARα. The PPARs are a
class of intracellular receptors that modulate carbohydrate, fat metabolism and adipose
tissue differentiation. Therefore activation of PPARα signaling results in (1) increased fatty
oxidation in the liver; (2) decreased hepatic triglyceride secretion; (3) increased lipoprotein
lipase activity and thus increased VLDL clearance; (4) increased HDL and many additional
effects.
Glucocorticoids
Glucocorticoids (GC) are a class of steroid hormones characterised by an ability to bind with
the glucocorticoid receptor (GR) and trigger similar effects. Glucocorticoids are distinguished
from mineralocorticoids and sex steroids by their specific receptors, target cells, and effects.
Cortisol (or hydrocortisone) is the most important human glucocorticoid. It is essential for life,
and regulates or supports a variety of important cardiovascular, metabolic, immunologic, and
homeostatic functions. Glucocorticoids are used as therapeutics to suppress various allergic,
inflammatory, and autoimmune disorders. They are also administered as posttransplantory
immunosuppressants to prevent the acute transplant rejection and the graft-versus-host
disease.
Thyroid hormones
The thyroid hormones, thyroxine (T4) and triiodothyronine (T3) are hormones produced by
the thyroid gland. An essential component in the synthesis is iodine. The major form of
thyroid hormone in the blood is thyroxine (T4). The thyroid hormones act on the body to
increase the basal metabolic rate, affect protein synthesis and increase the body's sensitivity
to catecholamines (such as adrenaline). The thyroid hormones are essential to proper
development and differentiation of all cells of the human body. These hormones also
18
regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic
compounds. They also stimulate the metabolism of vitamins. Both excess and deficiency of
thyroxine can cause disorders. T3 and T4 are used to treat thyroid hormone deficiency
(hypothyroidism). Synthetic forms of T3 and T4 like Liothyronine or Levothyroxine can be
used to replace thyroid hormones. Beside of this, natural thyroid drugs, made from
desiccated thyroid glands of pigs, are used in replacement therapy. They are both absorbed
well by the gut, so they can be given orally. Antithyroid drugs also called thionamides are
used to treat hyperthyroidism (overactivity of the thyroid gland) in order to reduce the
excessive thyroid activity. These drugs block the synthesis of thyroid hormones by the
thyroid gland and therefore decrease the production and blood levels of T4 and T3.
Antithyroid drugs require at least three weeks (usually six to eight weeks or longer) to lower
thyroid hormone levels because they only block synthesis of new T4 and T3; they do not
alter the effects of the T3 and T4 that are already present in the thyroid and the blood
stream. The two main antithyroid drugs currently available are: propylthiouracil (PTU) and
methimazole (MMI, Tapazole).
Anti-diabetic drugs
Anti-diabetic drugs treat diabetes mellitus by lowering glucose levels in the blood. There are
two different types of this disease. Diabetes mellitus type 1 caused by the lack of insulin and
therefore insulin must be used as therapy by injection or inhalation. Diabetes mellitus type 2
is caused by insulin resistance of the cells. Treatments include agents which increase the
amount of insulin secreted by the pancreas, agents which increase the sensitivity of target
organs to insulin and agents which decrease the rate at which glucose is absorbed from the
gastrointestinal tract.
Antidepressants
An antidepressant is a psychiatric medication or other substance (nutrient or herb) used for
alleviating depression. Drug groups known as monoamine oxidase inhibitors, tricyclics and
selective serotonin reuptake inhibitors are particularly associated with the term. Most
antidepressants have a delayed onset of action and are usually taken over the course of
weeks, months or years. Despite the name, antidepressants are often used in the treatment
of other conditions, including anxiety disorders, bipolar disorder, obsessive compulsive
19
disorder, eating disorders and chronic pain. Some have also become known as lifestyle
drugs or "mood brighteners".
The therapeutic effects of antidepressants are believed to be related to their effects on
neurotransmitters. Monoamine oxidase inhibitors (MAOIs) block the break-down of
monoamine neurotransmitters (serotonin and norepinephrine) by inhibiting oxidizing
enzymes, thus leaving higher levels still active in the brain.
Tricyclic antidepressants (TCAs) prevent the reuptake of various neurotransmitters, including
serotonin, norepinephrine, and dopamine. Selective serotonin reuptake inhibitors (SSRIs)
prevent more specifically the reuptake of serotonin thereby increasing the level of active
serotonin in synapses of the brain. Other novel antidepressants specifically affect serotonin
and other neurotransmitters.
Selective serotonin reuptake inhibitors (SSRIs)
Selective serotonin reuptake inhibitors (SSRIs) are a family of antidepressants considered to
be the current standard of drug treatment. It is thought that one cause of depression is an
inadequate amount of serotonin, a compound used in the brain to transmit signals between
neurons. SSRIs are said to work by preventing the reuptake of serotonin by the presynaptic
neuron. This family of drugs includes fluoxetine (Prozac), paroxetine (Paxil), escitalopram
(Lexapro, Esipram), citalopram (Celexa), and sertraline (Zoloft).
Serotonin-norepinephrine reuptake inhibitors (SNRIs)
Serotonin-norepinephrine reuptake inhibitors (SNRIs) such as venlafaxine (Effexor) and
duloxetine (Cymbalta) are a newer generation of antidepressant that works on both
norepinephrine and 5-hydroxytryptamine.
Noradrenergic and specific serotonergic antidepressants (NASSAs)
Noradrenergic and specific serotonergic antidepressants (NASSAs) form a newer class of
antidepressants which purportedly work to increase norepinephrine and serotonin
neurotransmission by blocking presynaptic alpha-2 adrenergic receptors, while at the same
20
time, minimizing serotonin related side-effects by blocking certain serotonin receptors. The
only example of this class in clinical use is mirtazapine (Avanza, Zispin, Remeron).
Norepinephrine (noradrenaline) reuptake inhibitors (NRIs)
Norepinephrine (noradrenaline) reuptake inhibitors (NRIs) such as reboxetine (Edronax) act
via norepinephrine (also known as noradrenaline). NRIs are thought to have a positive effect
on concentration and motivation in particular.
Norepinephrine-dopamine reuptake inhibitors
Norepinephrine-dopamine reuptake inhibitors such as bupropion inhibit the neuronal
reuptake of dopamine and norepinephrine (noradrenaline).
Tricyclic antidepressants (TCAs)
Tricyclic antidepressants are the earliest generation of these compounds and include
medications such as amitriptyline and desipramine. Tricyclics block the reuptake of certain
neurotransmitters such as norepinephrine (noradrenaline) and serotonin. They are used less
commonly now due to the development of more selective and safer drugs.
Monoamine oxidase inhibitor (MAOIs)
Monoamine oxidase inhibitors (MAOIs) such as phenelzine (Nardil) may be used if other
antidepressant medications are ineffective. Because there are potentially fatal interactions
between this class of medication and certain foods (particularly those containing Tyramine),
as well as certain drugs, classic MAOIs are rarely prescribed anymore. MAOIs work by
blocking the enzyme monoamine oxidase which breaks down the neurotransmitters
dopamine, serotonin, and norepinephrine (noradrenaline).
21
Psychoanaleptics
Psychoanaleptics, also known as stimulanting drugs that temporarily increase alertness and
awareness. They usually have increased side-effects with increased effectiveness, and the
more powerful variants are therefore often prescription medicines or illegal drugs.
Stimulants increase the activity of either the sympathetic nervous system, the central
nervous system (CNS) or both. Some stimulants produce a sense of euphoria, in particular
the stimulants which exert influence on the CNS. Stimulants are used therapeutically to
increase or maintain alertness, to counteract fatigue in situations where sleep must be
prevented (e.g. while operating vehicles), to counteract abnormal states that diminish
alertness consciousness (such as in narcolepsy), to promote weight loss (phentermine) as
well as to enhance the ability to concentrate in people diagnosed with attentional disruptions
(especially ADHD). Occasionally, they are also used to treat depression. The euphoria
produced by some stimulants leads to their recreational use, although this is illegal in the
majority of jurisdictions.
Anticonvulsants
The anticonvulsants, also called antiepileptic drugs (abbreviated "AEDs"), belong to a diverse
group of pharmaceuticals used in prevention of the occurrence of epileptic seizures.
Anticonvulsants are also increasingly used for the treatment of bipolar disorder, since many
seem to act as mood stabilizers. The goal of an anticonvulsant is to suppress the rapid and
excessive firing of neurons that start a seizure. Failing this, a good anticonvulsant would
prevent the spread of the seizure within the brain and offer protection against possible
excitotoxic effects that may result in brain damage. Many anticonvulsants block sodium (Na+)
channels, calcium (Ca2+) channels, AMPA receptors or NMDA receptors. Some
anticonvulsants inhibit the metabolism of GABA or increase its release. Two examples of
anticonvulants are described below.
Barbiturates
Barbiturates are drugs that act as central nervous system (CNS) depressants, and by virtue
of this, they produce a wide spectrum of effects, from mild sedation to anesthesia.
22
Benzodiazepines
The benzodiazepines are a class of drugs with hypnotic, anxiolytic, anticonvulsive, amnestic
and muscle relaxant properties. Benzodiazepines act as a central nervous system
depressant. Long-term use can be problematic due to the development of tolerance and
dependency.
Antineoplastic agents - Tumor chemotherapy
One of the challenges in today`s medicine is the fight against malignant tumors. Tumors are
responsible for about 25% and 29% of deaths in females and males, respectively, in
Switzerland, and similarly in other developed countries. The main characteristics of tumor
cells (in contrast to healthy cells) are:
•
they can grow independently from growth factors
•
they are resistant to inhibitory growth signals
•
they avoid apoptosis
•
they show unlimited replication
•
they induce agiogenesis
•
they have a tissue invasive potential.
Chemotherapy in its most general sense refers to treatment of disease by chemicals that
inhibit proliferation of cells, specifically those of microorganisms or tumors. In popular usage,
it usually refers to antineoplastic drugs used to treat cancer or the combination of these
drugs into a standardized treatment regime. There are two main ways to treat tumors;
surgery/radiation therapy for localized tumors and chemotherapy. There are different types of
chemotherapy, but the main goal of all of them is to inhibit proliferation of tumor cells or to
drive them to apoptosis.
Main groups of chemotherapeutics and mode of action
There are many diverse modes of actions of anticancer drugs. Whereas classical drugs
interfere with cell replication/division the new generation of drugs acts on angiogenesis and
cell signaling. Moreover, antihormonal drugs are applied in cancers dependent on hormones
for growth. The information to generate the categorization of cancer therapeutics was taken
23
from the book “Allgemeine und spezielle Pharmakologie und Toxikologie”, Urban und
Fischer, 9. Auflage (2004) if not further specified. The following types of drugs are currently
in use.
Alkylating antineoplastic agents
This group of drugs consists of chemicals that covalently bind to DNA bases, namely alkyl
groups, and as a consequence, the DNA strands are unable to uncoil and to separate.
Therefore the cell can no longer divide.
Examples of alkylating antineoplastic agents are:
•
Ifosfamide: it is used for the treatment of a variety of cancers including testicular
cancer, breast cancer and lung cancer. It is given intravenous.
•
Darcarbazine: it is used to treat malignant melanoma and Hodgkin lymphoma. The
administration to the patient is by injection or intravenous.
•
Busulfan: it is in clinical use since 1959 for the treatment of chronic myelogenous
leukemia. It is on the market under the brand name Myleran.
Platinum compounds
These agents interacalate with DNA strands and lead to the crosslinking of DNA strands
causing the inhibition DNA replication and cell death. Platinum therapy is used for the
treatment of specific cancers, including testicular, ovarian, lung, bladder, and head and neck
cancers. Platinum compounds currently used are cisplatin, its derivative carboplatin and
oxaliplatin, either used alone or in combination with other chemotherapy drugs. Oxaliplatin
belongs to a new class of platinum based compound and has been found to be effective in
various cancers that do not respond well to cisplatin.
Hydroxyurea
This compound inhibits the enzymes, which are responsible for the conversion of
ribonucleotidediphosphates to desoxyribonucleotidediphosphates, and as a consequence,
they inhibit the DNA synthesis. It is used for the management of melanoma, resistant chronic
myelocytic leukemia, and recurrent, metastatic, or inoperable carcinoma of the ovary.
24
Antimetabolites
An antimetabolite is a chemical with a similar structure to a substance (a metabolite) required
for normal biochemical reactions, yet different enough to interfere with the normal functions
of cells, including cell division. Antimetabolites can be used in cancer treatment, as they
interfere with DNA production and therefore cell division and the growth of tumors. Because
cancer cells spend more time dividing than other cells, inhibiting cell division harms tumor
cells more than other cells. Anti-metabolites masquerade as purine (azathioprine,
mercaptopurine) or pyrimidine - which become the building blocks of DNA. They prevent
these substances becoming incorporated into DNA during the S phase (of the cell cycle),
stopping normal development and division. They also affect RNA synthesis. However,
because thymidine is contained in DNA but not in RNA (where uracil is used instead),
inhibition of thymidine sythesis via thymidylate synthase selectively inhibits DNA synthesis
over RNA synthesis. Due to their efficiency, these drugs are the most widely used
cytostatics.
Examples of antimetabolites (purine analogues) are:
•
Thioguanine is used to treat acute leukemias and induction of remissions in acute
granulocytic leukemias
•
Pentostatin and cladribine are adenosine analogs which are primarily used to treat
Hairy cell leukemia
Microtubuli inhibitors
These compounds bind to the cytosceleton protein tubulin and inhibit the polymerisation of
tubulin to microtubuli. Therefore they inhibit the mitotic division of the cell, as chromosomes
remain in the aequatorial plate of the metaphase.
Examples of microtubuli inhibitors:
•
Vincristine and vinblastine are used to treat Hodgkin and non-Hodgkin lymphomas
and for acute lymphocytic leukemia
•
Paclitaxel (also known as taxol) is used to treat ovarian cancer, breast cancer and
lung cancer
25
Topoisomerase inhibitors
The nuclear enzymes topoisomerase I and II are critical for DNA function and cell survival,
they are responsible for the opening and closing of DNA strands during replication. As soon
as these enzymes are inhibited by drugs the cells can not divide anymore. Therefore, these
enzymes are promising cellular targets for anticancer drugs. Topoisomerase I inhibitors have
shown significant activity against a broad range of tumours. Because of manageable toxicity
and encouraging activity against solid tumors, topoisomerase I-active drugs offer promise in
the clinical management of human tumours. Topoisomerase II inhibitors are also active
against several types of tumors. However, treatment with these drugs often results in the
development of the multi-drug resistance.
Examples for topoisomerases inhibitors are:
•
Daunorubicin (belonging to the group of anthracyclines) is a topoisomerase II inhibitor
used to treat leukemia
•
Camptothecin is a topoisomerase I inhibitor and has a strong anticancer activity
Hormone-based tumor therapy
One category of hormone-based tumor therapy is build by the glucocorticoids. These drugs
are used for the treatment of lymphoma. Sex hormones, another category, are used to treat
prostate cancer and breast cancer. The growth of these cancers is often dependent on the
presence of sex hormones. One group consists of the anti-estrogen drugs, competitive
antagonist of the estrogen receptor. Another group is the anti-androgens which are applied to
treat prostate cancer. Another category of drugs used in hormone-based tumor therapy, are
the aromatase inhibitors. Aromatases are needed to convert androgens into estrogens and
the inhibition of responsible enzymes has an anti-estrogenic effect. Therefore tumors that
need estrogens for their growth will not proliferate. Aromatase inhibitors are increasingly
used in breast cancer and ovarian cancer therapy.
•
Prednison (glucocorticoid) is applied to treat leukemia.
•
Tamoxifen (anti-estrogen) is currently the most common hormone treatment used to
treat breast cancer. It is a competitive antagonist of the estrogen-receptor, which is
often expressed in tumor cells that need estrogens as a growth signal.
26
•
Flutamide, together with nilutamide and bicalutamide is the most used anti­
androgenic drug, used for prostate cancer hormone therapy. Flutamide fits into the
androgen receptors of the prostate cells and prevents the binding of testosterone,
and therefore it inhibits the growth of cancer cells.
•
Anastrozole, a drug belonging to the group of aromatase inhibitors, is used to treat
breast cancer.
•
Letrozole, another aromatase inhibitors, is used to treat postmenopausal women with
early-stage, hormone responsive breast cancer after surgery. Letrozole did better to
prevent a recurrence of disease - especially distant metastases - than the commonly
tamoxifen.
•
Exemestane is third member of the group of aromatase inhibitors that is often used to
treat women with breast cancer.
Inhibitors of angiogenesis
Angiogenesis, the formation of new blood vessels, is a process controlled by certain factors
(biomolecules). Some of them, like angiopoietin-1, basic fibroblast growth factor or vascular
endothelial growth factor, stimulate cells to repair damaged blood vessels or form new ones.
Other factors, called angiogenesis inhibitors, signal the process to stop. Angiogenesis plays
an important role in the growth and spread of cancer. Once a nest of cancer cells reaches a
certain size (1–2 mm in diameter), it must develop a blood supply in order to grow larger.
Diffusion is no longer adequate to supply the cells with oxygen and nutrients and to take
away metabolites. Therefore cancer cells secrete substances that promote the formation of
new blood vessels. The inhibition of angiogenesis is a promising target for anticancer drugs.
Examples for inhibitors of angiogenesis are:
•
Bevacizumab (also called Avastin) is used primarily in the treatment of colorectal
cancer, but also in other types of cancer. It binds to vascular endothelial growth factor
(VEGF), an important stimulator of angiogenesis, and stops the growth of new blood
vessels needed for the tumor to grow.
•
Suramin (also known as germanin) is used to treat prostate cancer. It inhibits
angiogenesis induced by vascular endothelial growth factor and basic fibroblast
growth factor.
27
Inhibitors of protein kinases
Protein kinases play important roles in regulating most cellular functions - proliferation/cell
cycle, cell metabolism, survival/apoptosis, DNA damage repair, cell motility, response to the
microenvironment - so it is no surprise that they are often themselves oncogenes. Kinases
such as c-Src, c-Abl, mitogen activated protein (MAP) kinase, phosphotidylinositol-3-kinase
(PI3K) AKT, and the epidermal growth factor (EGF) receptor are commonly activated in
cancer cells, and are known to contribute to tumorigenesis. There are several ways to target
these enzymes therapeutically, such as with antibodies or small molecules that block kinase­
substrate interaction. A number of kinase inhibitors have therefore already been developed
and approved for cancer treatment.
Examples for inhibitors of protein kinases are:
•
Imatinib, also known as Clivec is a tyrosine kinase inhibitor. It is used to treat
chronical myolid leukemia (CML). One hallmark of CML is the increased activity of the
Abl protein tyrosine kinase leading to proliferation of cells.
Imatinib inhibits the
activity of this kinase and reduces cell proliferation.
•
Gefitinib and cetuximab, inhibitors of the epidermal growth factor receptor, are used
for the treatment of lung and colorectal cancer. Gefitinib is a selective inhibitor of
epidermal growth factor receptor tyrosine kinase domain (EGFR). This target protein
is also known as Her1 or ErbB-1. EGFR is overexpressed in the cells of certain types
of human carcinomas leading to an inappropriate activation of the anti-apoptotic Ras
signalling cascade resulting in uncontrolled cell proliferation. Gefitinib inhibits EGFR
tyrosine kinase by binding to the adenosine triphosphate (ATP)-binding site of the
enzyme. Thus the function of the EGFR tyrosine kinase in activating the Ras signal
transduction cascade is inhibited.
Antibody based therapy
Human recombinant antibodies directed against proteins of cancer cells, although not tumor­
cell-specific however, are a new way to treat cancer. There are different ways how antibodies
fight against cancer cells. On one hand, antibodies make the cancer cell more visible to the
immune system. The immune system attacks foreign invaders in the body, but it does not
always recognize cancer cells as such. A monoclonal antibody can be directed to attach to
certain parts of a cancer cell, thus marking the cancer cell and making it easier for the
28
immune system to identify it. Another way of how antibody-based therapy works is the
inhibition of growth signals. Growth factors attach to receptors on the surface of normal cells
and cancer cells, signalling the cells to grow. Certain cancer cells express extra copies of the
growth factor receptor, thus leading to faster growth than in normal cells. Monoclonal
antibodies can block these receptors and prevent the growth signal from getting through.
Another method is the use of antibodies that can deliver radiation to cancer cells. By
combining a radioactive particle with a monoclonal antibody, radiation can be delivered
directly to the cancer cells. Instead of radioactive particles, also powerful anti-cancer drugs or
toxins can be attached to monoclonal antibodies and be delivered directly to the target
cancer cells.
Examples
for
antibody-based
therapy
are
(www.mayoclinic.com/health/monoclonal­
antibody):
•
The monoclonal antibody drug rituximab (Rituxan) attaches to a specific protein
(CD20) only found on B cells. Certain types of lymphomas arise from these same B
cells. When rituximab attaches to the protein on the B cells, it makes the cells more
visible to the immune system, which can then attack.
•
Cetuximab (Erbitux), a monoclonal antibody approved to treat colon cancer and
head and neck cancer, attaches to receptors on cancer cells that accept a certain
growth signal (epidermal growth factor).
•
Ibritumomab (Zevalin), approved for non-Hodgkin's lymphoma, combines a
monoclonal antibody with two radioactive particles. The ibritumomab monoclonal
antibody attaches to receptors on cancerous blood cells and delivers the radiation.
•
Gemtuzumab (Mylotarg), approved for treating a certain type of acute myelogenous
leukemia, is a monoclonal antibody attached to a potent anti-cancer drug derived
from a bacterium. The monoclonal antibody in gemtuzumab attaches to specific
receptors on leukemic cells. Then the anti-cancer drug enters the cancer cell and is
activated, causing the cancer cell to die.
Immunomodulators and Cytokines
Cytokines (either proteins or glycoproteins), secreted by immune cells, are the messengers
of the immune system. They have autocrine and paracrine functions, so that they function
locally or at a distance to enhance or suppress immunity. In cancer therapy, cytokines, which
29
enhance immunity, are used. Cytokines regulate cells like natural killer (NK) cells,
macrophages, and neutrophils belonging to the innate immune system. They also regulate
the cells of the adaptive immune system (T and B cell).
Examples of cytokines used in cancer therapy
•
Interleukin-2 (IL-2) and interferon-alfa 2b are two cytokines approved for the
treatment of cancer. IL-2 has demonstrated activity against renal cell, melanoma,
lymphoma, and leukemia. Interferon has activity in the same histologies but also in
Kaposi's sarcoma, chronic myelogenous leukemia, and hairy cell leukemia.
•
Interferon alpha, a family of molecules, binds to certain receptors on the surface of
immune cells. It has profound and diverse effects on gene expression. It promotes
the activity of B- and T-cells, and it also stimulates macrophages and dentritic cells. In
cancer therapy it is used to treat various cancers like kidney cancer, metastatic
melanoma, hairy cell leukemia and chronic myelogenous leukemia.
Environmental concentrations of antineoplastic drugs
There are over 50 cytotoxic drugs used routinely in chemotherapy in developed countries,
but there exist more of these compounds. Around 75% of these drugs are administered in
outpatient departments (patients come ambulant to see the doctor at the hospital) and the
rest given to inpatients (patients are stationary at the hospital) (Johnson et al., 2008). The
demand for chemotherapy treatment in developed countries continues to increase as cancer
incidence is increasing due to a greater proportion of elderly people in the population. The
majority of cytotoxic drugs are highly water soluble, with predicted logKOW between -1 and 3
(Pruijn et al., 2004). Many of the cytotoxic drugs appear to be poorly biodegradable when
incubated with activated sludge (Buerge et al., 2006). For example, Buerge et al. (2006)
found no difference in cyclophasphamide and 5-fluorouracil (both antimetabolite anti-cancer
drugs) concentration between influent and effluent in two Swiss sewage treatment plants.
Reasons for this lack of removal are: (1.) Many cytotoxic drugs are hydrophilic and will not
sorb to sludge, (2.) some of the antineoplastic drugs contain halogen atoms which are known
to be critical for biodegradation and (3.) many of the chemotherapeutics could be toxic for
sewage bacteria. Measurements for .cyclophosphamide and ifosfamide in sewage effluent
of plants which receive waste from hospitals showed concentrations up to 100 ng/L
(Kümmerer et al., 1997; Steger-Hartmann et al., 1997, 1996). In waters associated with a
Swiss sewage treatment plant 0.15-0.17 ng/L cyclophosphamide were detected (Buerge et
30
al., 2006). Zuccato et al. found 2-10 ng/L of cyclophosphamide in the river Lambro close to
Milan in samples collected 1997 (Zuccato et al., 2000). In conclusion, although fairly
persistent, only low concentrations (ng/L range) of chemotherapeutics can be detected in the
environment.
Potential effects of antineoplastic drugs on aquatic organisms
Many classical antineoplastic drugs used in cancer therapy have a high mutagenic and
cancerogenic potential. Parent compounds are often bioactivated leading to formation of
mutagenic metabolites. In case organisms in the environment are able to metabolize these
pharmaceuticals, enhanced mutation frequencies, and subsequently, a cancer risk will result.
But until now, it is uncertain that the activating enzymes for antineoplastic drugs are present
in all eukaryotic organisms. Current evidence suggests that this is probable at last in higher
eukaryotic organisms. One important goup of enzymes for the bioactivation of antineoplastic
drugs is the cytochrom P450 family (Kang et al. 2008). Cytochrom P450 can be found in
different vetebrate species (Fink-Gremmels 2008) and also in invertebrates (Snyder 2000). It
is questionable that they would be similarly activated in prokaryotic cells. In addition, these
drugs often have significant side effects in humans such as nausea, cytotoxicity, reduction in
proliferation of cells in various tissues etc. Such side effects can also occur in non­
mammalian species, as they have similar receptors (drug targets) than humans.
One would expect mutagenicity and cancerogenicity to occur in exposed aquatic organisms
as well. Ideally, information would be available from experiments, where fish are tested
against known concentrations of each cytotoxic drug of interest. However, to our knowledge,
no such studies have yet been published. Therefore it is necessary to utilize other relevant
information. It is well established that fish exposed to genotoxic xenobiotics develop DNA
damage (Barsiene et al., 2006; Bolognesi et al., 2006; Palhares et al., 2002), which in turn
can lead to an increased cancer incidence. In studies, where fish have been directly exposed
to cytotoxic agents, including 5-fluorouracil and cyclophosphamide, genetic damage has
been observed (Grisola et al., 2000; Grisolia, 2002). Therefore, it seems that cytotoxic drugs
will cause the same types of genetic damage in fish as in mammals, and maybe also in
invertebrates (Depledge, 1998). But it is currently unclear, which water concentration of
these anti-cancer drugs will cause DNA damage in fish and other aquatic organisms. For
estimation of potential toxic effects of antineoplastic drugs on aquatic organisms, already
known toxic data from non-aquatic vertebrates can be used for extrapolation to aquatic
organisms. It is known that 5-fluorouracil has the same toxic activity in rats (effects on
development at concentrations 10-30 mg/kg body weight in vivo and at concentrations 0.15­
31
0.3 µg/mL in vitro) (Kuwagata et al., 1998), hamsters (effects on development at
concentrations of 81 mg/kg body weight (Shah et al., 1989; Ben-Khaial et al., 1994), and in
the fruitfly Drosophila (effects at concentration of 0.13 µg/mL) (Parente et al., 1980; Rizki et
al., 1973) than in humans. Similar to these results, cyclophosphamide has been reported to
be active in a similar range of other organisms including the insects Drosophila (DNA
damage at concentration 140 µg/mL) (Siddique et al., 2005) and Anopheles (mutagenic
effects at 20.7 µg/mL) ((Anderson et al., 1995). It was shown that a ten days exposure of the
freshwater mollusk Biomphalaria glabrata with cyclophosphamide (3.6x10-4 M) and
mitomycin C (10-4 M) induces malformation of the offsprings (Nakano et al., 2003). Other
ecotoxicological data on aquatic organisms are lacking in the literature. Therefore there is a
need to perform pertinent experiments for use in risk assessment.
2. Identification of highly active pharmaceuticals
2.1 Identification of existing “however compounds”
One major challenge of the project was the identification of already existing “however”
compounds. To obtain this information we performed an extensive search in available
databases, summarized in Table 1, and existing literature.
Website
URL
PubMed
ScienceDirect
TOXNET
Scirus
ECOTOX
Drugsafety
http://www.ncbi.nlm.nih.gov/sites/entrez
http://www.sciencedirect.com/
http://toxnet.nlm.nih.gov/index.html
http://www.scirus.com/srsapp/
http://cfpub.epa.gov/ecotox/
http://drugsafetysite.com/
Table 1: Summary of used databases to identify “however” compounds.
Note that in the environmental risk assessment, the effect concentrations are divided by
assessment factors (security factors) to take into accout that extrapolations are made
between the laboratory and the environment, the variability within and between species, etc.
Therefore, the investigation was not limited to effect values lower than 10 ng/L, but also to
32
consider additional effect data at low concentrations. So far, the only identified substances
with effects at less than 10 ng/L are the synthetic sex hormone 17α-ethinylestradiol (EE2)
(and mestranol, the analogous compound) contained in contraceptive pills, and the
gestagene levonorgestrel (Table 2). Other compounds which exert adverse effects at very
low concentrations, but above 10 ng/L are listed in Table 2. Based on the mechanisms of
action and sorting the existing data we noticed that cancer chemotherapeutics may also be
of potential environmental concern, and therefore, potential “HC”. This group of drugs is
characterized by a specific mode of action affecting key biological targets with major
importance for the organism (e. g. cell proliferation, cell division, antihormonal action and cell
signalling disruption). Therefore small alterations of the function of these key targets induced
by chemotherapeutics can have major consequences on the organism. For example the
breast cancer therapeutics tamoxifen, an anti-estrogen that binds as antagonist to the
estrogen receptor, competes with the action of endogenous estrogens. This competition for
the receptor binding can be of environmental concern, because tamoxifen has the potential
to disturb the sex hormone system of aquatic organisms, because the homology between the
human estrogen receptor and the fish estrogen receptors is up to 90% (see Table 7). In a
partial full life cycle test with fathead minnow and the endpoints: development, growth and
reproduction, the NOEC was 5.12 µg/L (Williams et al., 2007). But as cancer
chemotherapeutics are regarded in a special report (Kümmerer et al., 2008), we did not go
deeper in this direction. In general, there are two groups of pharmaceuticals that may have
the potential to act as “HC”: endocrine disruptors and highly active/selective compounds that
act on key cellular targets. Endocrine disruptors (also referred to as hormonally active
agents) are exogenous substances that act like hormones and have the potential to disrupt
the physiological function of endogenous hormones. Since endogenous hormones are
typically present in the body at very low concentrations, the exposure to small amounts of
exogenous hormonally active substances can disrupt the proper function of the endocrine
system. An endocrine disruptor might have the potential to elicit adverse effects at much
lower concentrations than a toxic compound acting through an unspecific mechanism.
Therefore, endocrine active substances can have effects on aquatic organisms at low
concentrations. Examples for known endocrine disruptors are 17α-ethinylestradiol (EE2) and
methyltestosterone. EE2 can induce feminisation in fish at concentrations of 1-5 ng/L (Länge
et al., 2001). Additional data are summarized in Table 2. The androgen methyl testosterone
can lead to sex reversal in fish at concentration 100 ng/L (Fujioka, 2002). Sex reversal in fish
was also reported at concentrations of 10 ng/L (LOEC). The effect was elicited by a
combination of methyl testosterone together with a specific water temperature (Fujioka,
2002). Methyl testosterone induced imposex and altered spermatogenesis in snails at 100
ng/L (Schulte-Oehlmann et al., 2004).
33
Highly active compounds, which belong to the second group with the potential to be
“however” compound, are pharmaceuticals that have a specific mode of action and that
target specific human receptors or enzymes. As these targets can be also found in other
vertebrates, the risk to elicit an unwanted response in non-target species at low
concentrations exists. Examples for such pharmaceuticals are the aromatase inhibitor
fadrozole or the serotonin re-uptake inhibitor fluoxetine, both displaying unwanted effects on
fish at low concentrations. Cytochrome P450 aromatase CYP19, the target of fadrozole, is a
key enzyme in vertebrate steroidogenesis, catalyzing the conversion of C19 androgens to
C18 estrogens such as beta-estradiol (E2). Pimephales promelas exposed to fadrozole for
21 days showed reduction in fecundity at concentrations of 2 µg/L. Additionally, there was a
significant inhibition of brain aromatase activity in both male and female. In females, there
was a decrease in plasma E2 concentration (Ankley et al., 2002). Exposure of Oryzias
latipes to the serotonin reuptake inhibitor fluoxetine for 4 weeks lead to an increase in female
plasma estradiol levels at fluoxetine concentrations of 0.1 and 0.5 µg/L. Additional
developmental abnormalities were observed in offspring from fluoxetine exposed (0.1-5 µg/L)
parents (Foran et al., 2004). In Table 2 and 3 substances are listed with effects in aquatic
organisms at low concentrations. In addition to the endocrine active pharmaceuticals and
highly active/selective pharmaceuticals, antibiotics may induce adverse effects at low
concentrations. These effects concern in most cases plants and algae. For algae, the effect
values are in the low µg/L concentration range. To obtain the information for Table 2 and 3,
the databases PubMed and ScienceDirect were used. Some of the important key words for
the performed search were: endocrine disruptors, serotonin reuptake inhibitors and aquatic
organisms, sex reversal in fish, reproduction in fish and pharmaceuticals, aromatase
inhibitors and fish. The choice of the used key words was mainly based on own toxicological
knowledge.
Compound
Effect
LOEC
Organism
Reference
Effects below 10 ng/L
17α−Ethinylestradiol EE2
Feminisation
4 ng/L
Pimephales
Länge et al. 2001
promelas
(estrogen)
Fecundity, fertility
10 ng/L
34
Oryzias latipes
Hutchinson et al.
2003
Feminisation
Vitellogenin induction
5 ng/L
Tilapia
Shved et al. 2008
4 ng/L
Rutilus rutilus
Lange et al. 2008
2 ng/L
Danio rerio
Xu et al. 2008
1 ng/L
Pimephales
Jobling et al. 2003
promelas
1 ng/L
Increase in embryo
Jobling et al. 2003
antipodarum
production
Levonorgestrel
Potamopyrgus
Reproductive success
0.8 ng/L
Pimephales
Zeillinger et al. 2009
promelas
Effects at 10-100 ng/L
17-Methyltestosterone
100 ng/L
Sex reversal
Pimephales
Fujioka 2002
promelas
(androgen)
10 ng/L
Sex-reversal, but
Fujioka 2002
carassius
temperature dependent
100 ng/L
Imposex
Carassius
Marisa
Fujioka 2002
cormarietis
100 ng/L
Spermatogenesis
Clotrimazole (fungicide)
17 ng/L
Inhibition of algal 14α­
Marisa
Schulte-Oehlmann et
cormarietis
al. 2004
Mixed algal
Porsbring et al. 2009
population
demethylases
Effects at concentrations > 100 ng/L
Flutamide (antiandrogen)
Effects on
10 µg/L
Pimephales
Bayley et al. 2002
promelas
spermatogenesis
Courtship behaviour,
100 µg/L
Gasterosteus
aculeatus
nest building
35
Sebire et al. 2008
Sertraline (SSRI)
Reproduction
9 µg/L
Ceriodaphnia
Henry et al. 2004
dubia
Fluoxetine (SSRI)
Increased fecundity
56 µg/L
Ceriodaphnia
Brooks et al. 2003
dubia
Stimulation of
36 µg/L
Ceriodaphnia
Brooks et al. 2003
dubia
reproduction
Developmental
0.1-5
abnormalities
µg/L
Increased female
0.1 µg/L
Oryzias latipes
Foran et al. 2004
Oryzias latipes
Foran et al. 2004
Oryzias latipes
Paterson et al. 2008
Pimephales
Ankley et al. 2002
estradiol level
Longer half life than
0.64 µg/L
predicted from
mammalian data
Fadrazole (aromatase
Reduction in fecundity
2 µg/L
promelas
inhibitor)
Carbamazepine (anti­
Reproduction
25 µg/L
Ceriodaphnia
Ferrari et al. 2003
dubia
epileptic)
Sex ratio
10 µg/L
Daphnia magna
Flaherty et al. 2005
Diazepam (anxiolytic)
Regeneration
10 µg/L
Hydra vulgaris
Pascoe et al. 2003
Amlodipine (Ca-channel
Regeneration
10 µg/L
Hydra vulgaris
Pascoe et al. 2003
Regeneration
10 µg/L
Hydra vulgaris
Pascoe et al. 2003
Reproduction
0.5 µg/L
Oryzias latipes
Huggett et al. 2002
Elevated estradiol/
1 µg/L
Oryzias latipes
Huggett et al. 2002
Clofibric acid (anti­
hyperlipoproteinemic)
blocker)
Digoxin (digitalis
medicine)
Propranolol (β-Blocker)
36
testosterone levels in
males
Levofloxacin
Lomefloxacin
Growth inhibition
Growth inhibition
EC(10):
Plant
13 µg/L
(duckweed)
EC(10):
Plant
8 µg/L
(duckweed)
Brain et al. 2004
Brain et al. 2004
Ofloxacin
Growth inhibition
5 µg/L
Cyanobacteria
Ferrari et al. 2004
Sulfamethoxazole
Growth inhibition
5.9 µg/L
Cyanobacteria
Ferrari et al.
2003/2004
Triclosan
Altered development
EC(10):
Plant
11 µg/L
(duckweed)
Brain et al. 2004
0.15 µg/L
Xenopus laevis
Veldhoen et al. 2006
Table 2: Summary of pharmaceuticals exerting effects at aquatic organisms at low
concentrations, given are the lowest effect concentrations (LOEC).
In general, there are two groups of pharmaceuticals that may have the potential to act as
“however” compounds: endocrine disruptors and highly active/selective compounds that act
on key cellular targets. Endocrine disruptors (also referred to as hormonally active agents)
are exogenous substances that act like hormones and have the potential to disrupt the
hormone system.
Antibiotic
Levofloxacin
Lomefloxacin
Ofloxacin
Sulfamethoxazole
Triclosan
Effect
Growth
inhibition
Growth
inhibition
Growth
inhibition
Growth
inhibition
Altered
development
LOEC and effect
concentration
Organism
Reference
Brain et al. 2004
5 µg/L
Plant
(duckweed)
Plant
(duckweed)
Cyanobacteria
5,9 µg/L
Cyanobacteria
EC(10): 11 µg/L
Plant
(duckweed)
Xenopus laevis
EC(10): 13 µg/L
EC(10): 8 µg/L
0.15 µg/L
37
Brain et al. 2004
Ferrari et al.
2004
Ferrari et al.
2003/2004
Brain et al. 2004
Veldhoen et al.
2006
Table 3: Summary of antibiotics displaying effects on organisms at low concentrations, given
are the LOEC unless otherwise stated.
We hypothesize that only compounds with a specific mode of action can have critical
effects at such low environmental concentrations, because compounds with a more
general, unspecific mode of action display effects only at high environmental
concentrations. Another reason is that targets of pharmaceuticals may be identical or
similar in animals and lower organisms because receptors, biochemical pathways and
enzymes are conserved in evolutionary terms.
This holds true for nuclear steroid receptors that are very similar in organisms of different
evolutionary levels (Wilson et al., 2004), such as nuclear peroxisome proliferator-activated
receptors (PPAR`s) (Escriva et al., 1997), adrenoreceptors such as β1 and β2-receptors
(Nickerson et al., 2001). Also the insulin receptor, insulin-like growth factor and glucagon
receptors are present in lower vertebrates and invertebrates (Garofalo, 2002). Compounds
targeting these receptors could be “HC”, because compounds interacting with these
receptors induce important biological effects and necessary concentrations are very low.
2.2 Identification of important pathways of “HC”
As hormones play important roles in the development of organisms and in the regulation of
various metabolic pathways, we investigated first hormonal pathways and second additional
important pathways. We had a look at the working mechanisms, at the known homologies
between humans and other vertebrates/ invertebrates, at known pharmaceuticals interfering
with these pathways and at existing toxicological data. The information to make the following
categorization was obtained from the textbook “Allgemeine und spezielle Pharmakologie und
Toxikologie”, Urban und Fischer, 9. Auflage (2004), if not otherwise stated. A summary of
identified important pathways respectively receptors of “HC” is given in Table 4.
Compounds interacting with sex hormone system
In the following important hormones and their receptors are described along with their
evolutionary conservation, some of the important pharmaceuticals used affecting this
receptor system, and the potential ecotoxicological consequences.
Estrogen and the Estrogen receptor
Biology and physiology. Estrogens are a group of steroid compounds and function as the
primary female sex hormone. Like all steroid hormones, estrogens readily diffuse across the
38
cell membrane; inside the cell, they interact with estrogen receptors. The three major
naturally occurring estrogens in women are estrone (E1, produced during menopause),
estradiol (E2, predominate form in nonpregnant females), and estriol (E3, the primary
estrogen of pregnancy). In the body these are all produced from androgens through actions
of enzymes. Estrogen is produced primarily by developing follicles in the ovaries, the corpus
luteum, and the placenta. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
stimulate the production of estrogen in the ovaries. Some estrogens are also produced in
smaller amounts by other tissues such as the liver, adrenal glands, and the breasts. These
secondary sources of estrogen are especially important in postmenopausal women. While
estrogens are present in both men and women, they are usually present at significantly
higher levels in women of reproductive age. They promote the development of female
secondary sex characteristics, such as breasts, and are also involved in the thickening of the
endometrium and other aspects of regulating the menstrual cycle. In males, estrogen
regulates certain functions of the reproductive system important to the maturation of sperm
(Nilsson et al., 2001).
Estrogens bind to the estrogen receptor. There are two different forms of the estrogen
receptor, usually referred to as α and β, each encoded by a separate gene (ESR1 and ESR2
respectively). Hormone activated estrogen receptors form dimers, and since the two forms
are coexpressed in many cell types, the receptors may form ERα (αα) or ERβ (ββ)
homodimers or ERαβ (αβ) heterodimers. Estrogen receptor alpha and beta show significant
overall sequence homology, and both are composed of seven domains. Due to alternative
RNA splicing, several ER isoforms are known to exist. At least three ERα and five ERβ
isoforms have been identified (Li et al., 2004; Nilsson et al.,2001). Only in fish, but not in
humans, an ERγ receptor has been described (Hawkins et al., 2000). Different ligands may
differ in their affinity for alpha and beta isoforms of the estrogen receptor: 17-beta-estradiol
binds equally well to both receptors, estrone binds preferentially to the alpha receptor and
estriol to the beta receptor (Ascenzi et al., 2006).
Evolutionary conservation. Estrogen receptors have been identified in all vertebrate
groups from fish to mammals. The sequence homology between human and fish for example
is up to 90% (Tchoudakova et al., 1999).
Pharmaceuticals: Since estrogen circulating in the blood can negatively feed-back to
reduce circulating levels of FSH and LH, most oral contraceptives contain a synthetic
estrogen, along with a synthetic progestin. Estrogen and other hormones are given as
hormone replacement therapy to postmenopausal women in order to prevent osteoporosis
as well as treat the symptoms of menopause such as hot flashes, vaginal dryness, urinary
stress incontinence, chilly sensations, dizziness, fatigue, irritability, and sweating.
39
Ecotoxicological consequences. Due to the high conservation of estrogen receptors in
fish, exposure of fish to 17 α-ethinylestradiol to a concentration below 10 ng/L induces
feminisation and the expression of vitellogenin (Länge et al., 2001, 2008).
Progesterone and the Progesterone receptor
Biology and physiology. Progesterone (P4) is a steroid hormone that is synthesized by the
ovary, with the amount of P4 secreted depending on the level of gonadotropin stimulation
and the physiological status of the ovary. Moreover, granulosa cells, thecal/stromal cells, and
luteal secrete P4 albeit at different levels (Duleba et al., 1999). Once secreted from the ovary,
P4 acts at the level of 1) the hypothalamus-pituitary axis, to regulate gonadotropin secretion
and mating behaviour; 2) the mammary gland and 3) the uterus. P4 influences numerous
aspects of uterine physiology, including the differentiation of the endometrium and the
development of the placenta (Spencer et al., 2004). There are at least three progesterone
receptors that could regulate P4 action: PGR, members of the MPR family and PGRMC1.
PGR clearly plays essential roles in ovulation and the induction of genes associated with the
differentiation of luteal cells, and there are two potential mechanisms through which PGRs
could act. The first mechanism involves the well-characterized ligand induction of
transcriptional activity by the PGR (O'Malley, 2005). Recently, a second PGR-dependent
mechanism has come to light. In this mechanism P4 binding can activate the SH3 domain
within the PGR and, in turn, promote the binding and activation of Src kinases. By activating
Src kinase, P4 has the capacity to influence numerous signal transduction pathways that
ultimately regulate specific gene cascades (Shupnik, 2004). Members of the MPR family
have been shown to be also P4 receptors. MPRs are expressed in luteal cells (Cai et al.,
2005) and act to suppress intracellular cAMP levels and increase MAPK 3/1 activity (Zhu et
al., 2003). The net effect of MPR activation is unknown. However, because a decrease in
intracellular cAMP would suppress steroidogenesis (Peluso et al., 2004) and MAPK 3/1
activation is part of an apoptotic mechanism in many cell types (Peluso et al., 2005),
expression and activation of MPR in the luteal cells could initiate changes that would make
the luteal cell more susceptible to apoptosis and could promote the regression of the corpus
luteum. The third P4-regulated signal transduction pathway involves the interaction of
PGRMC1 and SERBP1. The role of SERBP1 in this complex is unknown but could involve
localizing PGRMC1 to the plasma membrane and/or influencing whether the high-affinity
and/or low-affinity binding sites within PGRMC1 are available to bind P4. PGRMC1 binds P4
40
and thereby promotes the activation of various kinases through an interaction with the Src
homology domains that are present within its cytoplasmic tail.
Evolutionary conservation. Sex steroids and their receptors have been demonstrated in all
vertebrate groups from agnatha to mammalia. The human sequence of the steroid hormones
receptor, including progesterone, show strong similarity to the homologs in amphibian and
fish (Baker, 1997). Recently, the presence of estrogen, androgen and progesterone in
gonads of amphioxus Branchiostoma belcheri (phylum: Chordata) was confirmed (Mizuta et
al., 2007).
Pharmaceuticals. Several synthetic ligands (both steroidal and non-steroidal) for PR have
been developed which compete for binding with the natural hormone and are capable of
inhibiting the receptor activity. Mifepristone (RU486) was the first of these PG antagonists
that exhibited antiprogesterone activity in humans and has since been used in numerous
clinical studies in the gynecologic and obstetrical fields. It was shown to be an effective
abortifacient and postcoital contraceptive and has been used in the treatment of
endometriosis, uterine myomas, and for meningiomas, which have large concentrations of
PRs. Progestogenic hormones (progestins) are a component in oral contraceptives and
hormone replacement therapies. Among others three progestins are currently marketed in
contraceptive formulations, namely levonorgestrel, dienogest and drospirenolone.
Ecotoxicological consequences. Due to the sequence similarity between human and
animal steroid hormone receptors, fish are also a target for human pharmaceuticals
interacting with the progesterone receptor. It was shown, that Mifepristone inhibits the
glucocorticoid receptor of rainbow trouts at a concentration of 14 µM (Mazon et al., 2004).
The progestins lebonorgestrel, dienogest and drospirenolone caused an inhibition of
reproductive success and hisological changes in gonads (Zeilinger et al., 2009).
Levonorgestrel acted at extremely low concentrations of 1 ng/L, and dienogest at 6.5 µg/L.
Androgen and the Androgen receptor
Biology and physiology. Androgens play an important role not only in male sexual
differentiation, puberty, sexual behavior and spermatogenesis, but also in the maintenance of
bone architecture and muscle mass and strength. Androgen receptors (AR) belong to a
superfamily of nuclear hormone receptors. They play an important role in the development
and maintenance of male organs. These functions are largely attributed to the transcription
factor functions of AR mediating the physiologic effects of androgens. By binding to specific
41
DNA sequences known as androgen-responsive elements (AREs), AR modulates the
transcription of androgen-responsive genes. AR is nearly ubiquitous in mammalian tissues
sequestered and stabilized in the cytoplasm bound to heat shock proteins. Upon binding
testosterone or dihydrotestosterone, it undergoes a series of conformational changes that
lead to translocation to the nucleus, allowing interactions with androgen response elements
at various androgen target genes (Gelmann, 2002).
Evolutionary conservation. The AR genomic organisation is conserved throughout
mammalian evolution from rodents to man. Conservation of segments of the AR gene
throughout evolution implicates these regions as being critical for the activity of the molecule.
The DNA binding domain of the AR is most highly conserved from Xenopus to human, and
the ligand binding domain of the AR is also highly conserved.
Pharmaceuticals. Synthetic agonistic or antagonistic ligands of the AR are used in hormone
therapy to treat hypogonadal men, osteoporosis, frailty and prostate cancer. Beside of this,
steroidal androgens are abused by athletes as anabolic agents for enhancing physical
performance. The major goal of androgen therapy is to achieve testosterone levels as close
to physiological concentrations as possible. Methyltestosterone, an AR agonist, is used to
treat men with testosterone deficiency and also to treat breast cancer, breast pain and
swelling due to pregnancy in women. Flutamide, an oral antiandrogen drug, is primarily used
to treat prostate cancer. It competes with testosterone and its metabolite dihydrotestosterone
for the binding to the AR in the prostate gland. A newer antagonist of the AR is bicalutamide
that has compared to flutamide a better side-effect profile.
Ecotoxicological consequences. Beside of binding to the human AR, synthetic androgens
can bind to animal AR. Flutamide exposure (50-500 µg/l) of female fathead minnows induces
an decrease in mature oocytes and exposure of male fathead minnows to the same
flutamide concentrations induces degeneration and necrosis of spermatocytes (Jensen et al.,
2004). Exposure of juvenile guppies to 10 µg/l flutamide results in the reduction in number of
sperms. Methyltestosterone leads to masculinisation and intersex of Korean rockfish feeded
with 0.05 µg/g (Bayley et al., 2002).
Thyroid Hormones
Biology and physiology. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are
essential for growth and differentiation, for the regulation of energy metabolism, and for the
physiological function of virtually all human tissues. The production of thyroid hormone is
42
regulated by the classic hypothalamus–pituitary–thyroid axis. T4 and T3 are secreted to the
blood stream from the thyroid gland in response to thyroid stimulating hormone (TSH),
produced by thyrotropes in the pituitary, which is released in response to thyrotropin
releasing hormone (TRH) from the hypothalamus. Once secreted into the blood stream, THs
are reversibly bound to plasma binding proteins (thyroxine-binding globulin, thyroxine-binding
prealbumin or transthyretin, and albumin), which facilitate TH transport and entry into tissues
including brain. T3 is the transcriptionally active TH, which is produced intracellularly from
circulating T4 by the process of deiodination. Deiodination is the stepwise enzymatic removal
of iodine from the TH molecule by enzymes known as deiodinases (Eales et al., 1993). Once
in the cell, T3 moves to the nucleus and binds to intracellular thyroid hormone receptors
(TRs). TRs belong to the nuclear hormone receptor superfamily (Yen, 2001).
In mammals there are two main TR subtypes, TRα and TRβ, which are encoded by different
genes. The TR binds the TRE (thyroid response elements) in the promoter of thyroid target
genes generally as a heterodimer with a 9-cis retinoic acid receptor (RXR). Unliganded
TR/RXR bound to a positively regulated TRE represses transcription by recruitment of co­
repressor molecules, whereas once ligand binds to the TR/RXR, transcription is activated by
recruitment of co-activator molecules. The opposite effect occurs on negatively regulated
TREs (Yen, 2001).
Evolutionary conservation Thyroid hormones and thyroid hormone receptors can be found
in all vertebrates. Beside of this, thyroid hormone receptor orthologues have been found in
Plathylminths, Mollusca, Crustacea and Echinodermata (Wu et al., 2007).
Pharmaceuticals. To treat thyroid gland malfunction, several pharmaceuticals are on the
market. Levothyroxine is one of the most prescribed thyroid hormone replacement drug,
used to treat hypothyroidism. It is a synthetic form of the natural T4. Another frequently used
drug is liothyronine, a synthetic form of the T3. Additionally, natural thyroid drugs are made
from the desiccated thyroid glands of pigs. To treat hyperthyroidism, antithyroid drugs like
methimazole are on the market. This compound inhibits the synthesis of thyroid hormones.
Ecotoxicological consequences. Polychlorinated biphenyls (PCBs), hydroxylated PCBs,
dibenzo-p-dioxins and dibenzofurans interact strongly with the mammalian TH transport
protein, transthyretin (TTR). The interactions of these endocrine disrupting chemicals (EDCs)
with TTR induced a rise in the plasma clearance rates of THs, inducing hypothyroxinemia in
rat, seal and human (Crofton et al., 2005). The most characteristic feature of TTRs from non­
mammalian vertebrates, such as birds, amphibians and fish, is their higher affinity for T3 than
for T4 (Yamauchi et al., 1999). Therefore, chemicals interfering with T3-binding to TTR may
43
directly affect the free concentration of plasma T3 and the plasma clearance rate of T3 in
non-mammalian vertebrates.
The antibacterial compound triclosan (TCS) is a widespread environmental contaminant
found in wastewater effluents (0.01 – 0.65 µg/l) in the US, UK, Japan and other countries. It
has structural similarity to thyroid hormones. At high concentrations (up to 230 µg/l were
used in this study) TCS has negative effects on tadpole activity and on final mass of
Xenopus laevis (Fraker et al., 2004). At environmental relevant concentrations, TCS alters
the rate of T3-induced metamorphosis. Veldhoen et al pretreated X. laevis tadpoles with 0.15
µg/l TCS for 4 days, afterwards they injected the thyroid hormone T3. With this treatment,
they observed increased hind limb development and a decrease in total body weight.
Additionally, they showed an altered TH receptor mRNA expression in X. laevis cells treated
with 0.03 µg/l TCS before T3 treatment (Veldhoen et al., 2006)
Corticoid agonists and antagonists
Biology and physiology. The group of nuclear steroid receptors includes the corticosteroid
receptors and their ligands. These ligands are steroid hormones, produced in the adrenal
cortex from the precursor cholesterol. They display an important role in mediating stress
responses, in regulating the immune response, anti-inflammatory processes, blood
electrolyte levels and carbohydrate metabolism. There are mainly two groups of
corticosteroids, the glucocorticoids with cortisol as one prominent member and the
mineralcorticoids with aldosterone as one member. They are responsible for the control of
the carbohydrate, fat and protein metabolism, respectively. They control the electrolyte and
water levels, mainly by promoting sodium retention in the kidney. Important enzymes for the
synthesis of corticosteroids out of cholesterol are the enzymes from the cytochrome P450
superfamily. The corticoid receptors form a complex with other proteins such as heatshock
proteins in the cytosol. In this complex, the receptors are inactive. As soon as a
corticosteroid enters the cytosol, it binds to the receptor. This binding induces a
conformational change of the receptor, the complex comes apart and the free receptor
translocates to the nucleus. There the receptor can function as a transcription factor by
binding to the DNA and inducing gene transcription. Beside of this, the receptor can inhibit
gene expression through interaction with other transcription factors.
Evolutionary conservation.
Similarly to what
has
been reported in mammals,
corticosteroids are major endocrine factors in teleost, involved in regulation of physiological
functions, such as osmoregulation, respiration, immune responses, reproduction, growth,
44
and metabolism (Mommsen et al., 1999). The major corticosteroid released from the teleost
interrenal is cortisol although, in a few species, small amounts of corticosterone, and other
18-hydroxylated steroids have been observed. There is a close similarity between fish and
mammalian GR and MR (Fig.4). Due to the homology in the corticoid receptors, higher fish
are a good target for human pharmaceuticals like corticoid receptor agonists and
antagonists.
Pharmaceuticals. Based on the structure of cortisol, there are synthetic corticosteroids as
pharmaceuticals on the market. The synthetic steroids display a higher affinity to the
corticoidreceptor than the natural occurring ligands. These drugs are used as anti­
inflammatory and immunosuppressive compounds. Beside these synthetic agonists, there
are also some synthetic antagonists on the market.
Ecotoxicological consequences. It was shown that the synthetic agonist dexamethasone
binds to the fish glucocorticoid receptor 1 and 2 with EC50 values of 1.4 nM and 0.35 nM
(Bury et al., 2003). This group of pharmaceuticals could be a candidate to be “HC”.
Fig.4: Amino acid identity between selected domains (A/B, C, D, and E) of the trout MR
(rtMR) and human MR (hMR), (Prunet et al., 2006)(MR: mineralocorticoid receptor)
Parathyroid hormones
Biology and physiology. Parathyroid hormone (PTH) and PTH-related protein (PTHrP) are
two factors that share amino acid sequence homology and act via a common receptor. In
tetrapods, PTH is the main endocrine factor acting in bone and kidney to regulate calcium
and phosphate. PTHrP is an essential paracrine developmental factor present in many
tissues and is involved in the regulation of ossification, mammary gland development, muscle
relaxation, and other functions. The actions of PTH and PTHrP are mediated by a G protein
coupled receptor, referred to as PTH receptor 1 (PTHR1). Ligand binding to PTHR 1
stimulates G-mediated activation of adenylyl cyclase which stimulates cAMP production, and
45
subsequent activation of protein kinase (PKA). PTHR 1also stimulates G-mediated activation
of protein kinase C (PKC).
Evolutionary conservation. Fish apparently lack an equivalent of the parathyroid gland,
and therefore the existence of PTH-like factors in fish has been the subject of debate.
However, final confirmation for their presence in fish came from the isolation of cDNAs for
functional PTH receptors in zebrafish Danio rerio (Rubin et al., 1999). So far two forms of
PTH, two of PTHrP and a protein with intermediated characteristic have been identified in
teleost fish. Piscine and tetrapod PTHrP share high sequence homology at the N-terminus
and its function in calcium metabolism also appears to have been conserved in vertebrates.
Accessorily immunoreactive PTH-like peptides were detected in invertebrate ganglia (in the
pond snail Lymnea stagnalis) (Hull et al., 2006)
Pharmaceuticals. Synthetic parathyroid hormones are used to treat osteoporosis.
Intermittent administration of parathyroid hormone stimulates bone formation by increasing
osteoblast number and reduces the incidence of fracture in postmenopausal women, in
elderly men, and in women with glucocorticoid-induced osteoporosis. Preotact belongs to
this group of pharmaceuticals, and its structure is very close to the structure of natural
occurring parathyroid hormones.Calcium is essential for a number of vital processes, from
bone formation to blood clotting, regulation of enzymatic processes and modulation of
permeability and excitability of plasma membranes. The physiological relevance of calcium
requires that its concentrations in extracellular fluid are tightly controlled within a narrow
range and mammalian species have developed a complex homeostatic system that includes
parathyroid glands, kidneys, intestine and bones. Fish also maintain their circulating calcium
levels within controlled limits, although the systems involved in this regulation are not totally
clarified. In teleost fishes, a considerable part of the body calcium is stored in the scales,
which are hard, generally flattened, skeletal elements found in the skin. Human
pharmaceuticals interfering with the parathyroid receptors could alter this tightly regulated
calcium homeostasis, and therefore they could be candidates for being “HC”.
Pharmaceuticals interacting with the cytochrome P450 system
Biology and physiology. The cytochromes P450 comprise one of the largest and most
versatile protein families. P450 enzymes are particularly known to metabolize a variety of
lipophilic xenobiotics e.g. drugs, pesticides, polycyclic aromatic hydrocarbons and plant
allochemicals (Gonzalez., 2004). Many P450s have, however, endogenous functions, being
specialized in the metabolism of signal molecules, such as steroid hormones (Werck­
46
Reichhart et al., 2000). P450s regulating physiologically important reactions usually have
narrow substrate specificity and are often conserved between species and sometimes phyla
to preserve important biosynthetic pathways. In contrast, many P450s that metabolize
exogenous compounds have broader substrate specificities. Several metazoan genomes are
now sequenced, revealing the presence of approximately 50–150 different P450 genes,
including some pseudogenes, in species so distinct as nematodes, arthropods and
mammals. Although the primary sequence identity between P450 proteins often is below
20%, highly conserved residues of the canonical P450 motifs ensure that the structural fold
of the protein remains conserved (Werck-Reichhart et al., 2000).
Evolutionary conservation In marine invertebrates P450 genes and biochemical evidence
of P450 enzyme activity have been demonstrated in Cnidaria, Annelida (Polychaeta),
Mollusca, Arthropoda (Crustacea) and Echinodermata (Lee, 1998).
Pharmaceuticals. Human pharmaceuticals that affect the cytochrome P450 system, can
display also effects on this system in lower vertebrates and invertebrates. Examples of such
pharmaceuticals are:
•
Barbiturates: Induction of cytochrome P450 enzymes in the liver
•
Macrolide antibiotics: inhibit CYP3A4, a member of the cytochrome P450
•
Inhibitors
of
the
viral
reverse
transcriptase
like
Delaviridin:
inhibit
the
monooxygenases CYP3A and CYP2D6
•
Inhibitors of the cytochrom P450 aromatase (CYP19): inhibit synthesis of estrogen
Ecotoxicological consequences. Such compounds have the potential to be “however
compounds”. For example it was shown that the CYP19 inhibitor fadrozole displays effects
on the fathead minnow (Pimephales promelas) reproductive endocrinology and physiology in
a short-term reproduction assay. A concentration-dependent reduction in fecundity was
observed in fish exposed for 21 days to water concentrations of fadrozole ranging from 2 to
50 µg/l. There was a significant inhibition of brain aromatase activity in both male and female
fathead minnows exposed to fadrozole at concentration of 50 µg/l. In females, this inhibition
was accompanied by a concentration-dependent decrease in plasma E2 and vitellogenin
concentrations at fadrozole concentrations of 10 µg/l and 2µg/l (Ankley et al., 2002).
47
Pharmaceuticals showing mutagenic activity
Zidovudine (AZT), lamivudine (3TC), and abacavir (ABC) are nucleoside reverse
transcriptase inhibitors (NRTIs) often used as components in ‘‘highly active antiretroviral
therapy’’ (HAART) designed to inhibit viral replication in HIV-infected patients. NRTIs are
analogs of normal nucleosides that lack the 3`-OH of the deoxyribose sugar, and are
therefore unable to extend the nascent DNA chain by forming a 5` to 3`-phosphodiester bond
with the proceeding nucleic acid. They are able to inhibit viral replication by taking the place
of endogenous nucleotides during reverse transcription of viral RNA and, thus, cause
premature termination of proviral DNA synthesis. However, this mode of action also allows
for the incorporation of these drugs into host cell nuclear DNA and mitochondrial DNA.
Torres et al investigated the mutagenicity of AZT, 3T3 and ABC on human TK6
lymphoblastoid cells. They could show that exposure of the cells to 33 µM ABC increases the
mutant frequency. AZT and 3T3 in a combined treatment, increases the mutant frequency
also at 33 µM (Torres et al., 2007)
Immunosuppressive drugs such as cyclosporine A, mycophenolate mofetil, tacrolimus, and
the immunosuppressive agent sirolimus are used effectively to prevent immunologic rejection
after solid-organ transplantation. The most serious complication among patients undergoing
immunosuppressive therapy is the risk of developing cancer. Oliveira et al investigated the
mutagenicity of these compounds in human lymphocyte cultures. Addition of cyclosporine A
to cultures led to a rise in mutagenicity at concentrations of 200 µg/L and 400 µg/L. The
immunosuppressive drugs, sirolimus induced mutagenic activity at concentration 50 ng/mL
(Oliveira et al., 2004)
This group of pharmaceuticals could also display effects at aquatic organisms, specially on
such species which are exposed to this compound during their whole life.
Receptor
Ligand
Biological Functions
Hormone receptors
Estrogen receptor α, β,( γ: only
Estrogen
primary female sex hormone,
promotes development of
in fish)
female, secondary sex
characteristics
48
Progesteron receptor
Progesterone
involved in female menstrual
cycle and pregnancy,
embryogenesis of humans and
other species
Androgen receptor
Testosterone
principal male sex hormone,
stimulates muscle growth and
bone density
Thyroid hormone receptor α, β
Thyroid hormone
important for
development/differentiation of
cells, regulation of protein, fat
and carbohydrate metabolism
Glucocorticoid receptor
Cortisol
involved in stress response,
increases blood pressure, blood
sugar levels,
immunosuppressive action
Mineralocorticoid receptor
Aldosterone
reabsorption of sodium ions in
the kidney, secretion of
potassium ions, increase of the
blood pressure
Retinoic acid receptor RAR
Vitamin A and related
form dimers with other nuclear
α β, γ and retinoid X receptor
compounds
receptors
important for development
RXR α, β, γ
Other but minor
important receptors
Peroxisome proliferator­
free fatty acids
form dimers with other nuclear
activated receptor PPAR
receptors like RXR, role in fat
α, β, γ, δ
metabolism
α-Adrenoreceptor
Noradrenalin
smooth muscle contraction
49
β-Adrenoreceptor
Adrenalin
Increasing the heart beating
rate,mobilizing energy
GABA receptors
Neurotransmitter
inhibitory effects in CNS
GABA
Table 4: Important receptors and their ligands, which may be potential targets of “HC”.
3. Concepts for identification of “HC”
As already mentioned before, we think that only pharmaceuticals with the specific mode of
action can elicit effects at concentrations close to 10 ng/L. Therefore we developed a
concept based on the mode of action. Additionally we discuss, whether two other already
existing approaches the fish plasma model concept and a QSAR concept, might be helpful in
identifying compounds with effects at low concentrations.
3.1 Mode of action concept
The mode of action of pharmaceuticals can provide useful information concerning the
potential toxic effects on environmental targets. In contrast to compounds acting non­
specifically via a narcosis-type mechanism, drugs acting via specific mode of actions are
potent enough to elicit responses at very low concentrations.
3.1.1 Consideration of toxicological data
Before considering the mode of action concept in detail, we wanted to gain a closer look at
the existing toxicological data of pharmaceuticals that are generated during the registration
process of each new drug. During the evaluation process of a new pharmaceutical,
toxicological studies are performed to analyse for the potential risks of a new drug with mice
and rats, and sometimes dogs. Such acute and chronic experiments performed for the
registration of drugs are directed to clarify the three main questions: 1.Which doses are
lethal? 2.
Which doses elicit adverse chronic effects? 3. What are the chronic effects
including reprotoxicity, mutagenicity, carcinogenicity and teratogenicity? These data may be
an important source of information for the identification of the modes of action and the
evaluation of possible targets not only in toxicology, but also in ecotoxicology.
50
Due to the lack of chronic data concerning the effects of pharmaceuticals on aquatic
organisms, it is important to have a closer look on the existing toxicological data generated in
toxicological experiments and studies with animals during the non-clinical testing and healthy
volunteers or patients during the clinical evaluation. These data can possibly be used to
identify targets and possible toxicological effects of the pharmaceuticals in question on
aquatic organisms. Due to the lack or incompleteness of chronic toxicological data published
on aquatic or terrestrial organisms, such toxicological data in mammals are valuable for
potential use in ecotoxicological risk assessment.
Use of toxicological data for identification of “HC” compounds
During
the
non-clinical
evaluation
of
new
pharmaceuticals
the
pharmacology,
pharmacokinetic and toxicology of the new drug is evaluated. Toxicological data serve as a
basis for predicting potential adverse effects in humans. The onset, severity and duration of
the toxic effects, their dose-dependency and degree of reversibility (or irreversibility), and
species or gender-related differences should be evaluated during the non-clinical testing.
Important endpoints of the non-clinical testing are:
•
Pharmacodynamics
•
Toxic signs
•
Causes of death
•
Pathological findings
•
Genotoxic activity
•
Carcinogenic potential
•
Carcinogenic risk for humans
•
Fertility and embryofetal development
•
Effects on juvenile animals
One important outcome of such toxicological testing is the relation between the no observed
adverse effect levels (NOAEL) and the toxic dose in relation to the maximum recommended
human dose. Both, the NOEAL or the toxic dose can give useful information for the risk
assessment. If the toxic dose of a pharmaceutical is close to its environmental concentration,
there is a high risk that the pharmaceutical elicits unwanted effects in the environment.
Taking into account that for most pharmaceuticals the environmental concentration is not
known, the predicted environmental concentration PEC has to be taken for the risk
assessment. The studies to determine the toxicity of a new pharmaceutical are mainly
performed in-vivo in mice, rats, dogs, in some cases in non-human primates. Some tests are
51
performed in-vitro in cell culture systems (Table 5). Another point of such studies is the route
of administration. For ecotoxicological assessments, the optimal case would be that the route
mimics the way, how animals may be exposed to the drug in the environment. But also if this
is not the case, the toxicological data can give useful hints for the risk assessment.
Application for ecotoxicological risk assessment
These data are important for the toxicological characterisation of drugs, particularly for
identifying critical targets of effects (Table 6). However, an extrapolation of these
toxicological data to aquatic organisms is difficult to perform. One major problem is that often
only high doses are used in such experiments. They are justified to answer regulatory needs,
but the problem of chronic effects at low concentration on aquatic organisms usually cannot
be satisfactorily answered. Therefore, data from toxicological and pharmacological
characterization of drugs are important to give hints for potential targets in organisms others
than mammals. They may also point to the modes of action of these drugs. Pharmaceuticals
having a toxic concentration or a NOAEL close to the concentration expected to occur in the
environment (or PEC) must be considered as high risk compounds. However, target organs
and potential toxicity in aquatic life can only be assessed by specific experiments with
aquatic organisms.
Toxicology
Non-clinical
guidelines*
Single dose
toxicity
3BS1A
Repeated dose
toxicity
ICHS4A
Type of study
Endpoints
Single-dose oral
study in mice
Single-dose oral
study in rats
Single-dose
intravenous study
in mice
Single-dose
intravenous study
in rats
Two-week oral
range finding study
in rats
Five-days oral
range-finding study
in monkeys
Three-month oral
study in rats
One-month oral
study in dogs
- Observed maximal non-lethal
dose
- Approximate lethal dose
- Body weight
- Noteworthy findings
- Clinical observations
52
- No observed adverse effect
level NOAEL
- Body weight
- Clinical observations
- Died/sacrificed moribund
- Food consumption
- Hematology
- Serum chemistry
- Organ weights
- Histopatology
Nine-moth oral
study in dogs
Two-weeks
palatability study in
rats
Three-month
dietary range­
finding study in
mice
Six-month oral
study in rats
Ames Test
Human
lymphocytes assay
in-vitro
Micronucleus test
in rats
Genotoxicity
ICHS2,2A,2B
Carcinogenicity
ICHSIB,SIA,S1C
Dietary
carcinogenicity
study in mice
Oral
carcinogenicity
study in rats
Reproductive and
developmental
toxicity
ICHS5A
Embryo-fetal
development study
in rats
Embryo-fetal
development study
in rabbits
Fertility and early
embryonic
development study
in rats
In-vitro: induction of reverse
mutations in bacterial cells;
- Induction of chromosome
aberrations
In-vivo: induction of bone­
marrow micronuclei;
- Induction of unscheduled
DNA synthesis
- Died/sacrificed moribund
- Survival
- Body weight
- Food consumption
- Number of animals with
neoplastic lesions in different
tissues, for example
hepatocellular adenoma
- Fertility:
number of sperm positive
females number of pregnant
females number of
implantations, percentage of
pre- and post-implantation loss
- Early embryonic
development/ Embryo-fetal
development:
- Number of litters, number of
live fetuses and of dead
fetuses, mean fetal body
weight, fetal sex ratio, fetal
abnormalies
- Prenatal/postnatal
development
Table 5: Summary of recommended toxicity tests during the non-clinical evaluation of new
pharmaceuticals according to the common technical document for the registration of
pharmaceuticals for human use, international conference on harmonisation of technical
requirements for registration of pharmaceuticals for human use, 2002. Noteworth findings
range from mild and moderate to marked. * Non clinical guidelines for human medicinal
products
according
to
the
European
Medicines
www.emea.europa.eu/htms/human/humanguidelines/nonclinical/htm
53
Agency;
The most critical endpoints of the toxicity tests for the identification of potential “HC” are:
•
Death
•
Effects on hormone system
•
Effects on cell proliferation
•
Alterations in the reproduction
•
Developmental defects
All theses endpoints are critical for the individual because they affect important functions like
survival. Beside the importance for the single organism some endpoints like reproduction and
development are important for the whole population. As soon as the tested pharmaceuticals
display adverse effects at these endpoints at concentrations close to the PEC, they are at the
risk to be “HC”. But it is difficult to define a threshold or border area. Table 6 summarizes all
important endpoints of the toxicological tests for the identification of potential “HC”.
Test/test number
Important endpoints
Single dose toxicity > Death
3BS1A
> Organ toxicity
> Effects on hormone
system
Repeated
dose > Death
toxicity ICHS4A
> Organ toxicity
> Critical targets:
hormone system, cell
proliferation
Reproduction/
> Reduced reproduction
> Changed ratio males
Development
to females
ICHS5A
> Reduced viability or
higher mortality of the
off spring
> Developmental
defects
Carcinogenicity
> Development of
ICHSIB,SIA,S1C
cancer
Actions
> If highly acute toxic than
high probability to be “HC”
> If highly acute toxic than
probability to be “HC”
> If hormone system/organs
are affected, than high
probability to be “HC”
If fertility and reproduction
affected, than high
probability to be “HC”
> Should be compared to
other data on fertility and
reproduction
Table 6: Identification of pharmaceuticals with high probability to be “HC” on the basis of
toxicological data (important endpoints of EMEA-tests).
54
3.1.2 Description of the mode of action concept
Evolutionary receptor conservation and homologies
A specific mode of action includes, among others, binding of a drug to a specific human
receptor. This binding can have agonistic or antagonistic effects on metabolic/endocrine
reactions controlled through the targeted receptor. Interactions of drugs with specific target
enzymes are an additional important mode of action leading to activation or inhibition of the
enzyme. As a consequence this leads to activation or inhibition of certain metabolic
pathways. A third mode of action is the interaction with cellular structures and biomolecules
such as DNA, RNA, lipids and proteins. Compounds applied in cancer therapy are
specifically directed to such targets in addition to their effects on cell signalling and cell
replication. Oxidative stress is also directed in an unspecific manner to such target
molecules.
Here we propose a pragmatic and stepwise approach based on the mechanisms of actions
of drugs. In a first step of our concept to identify “HC”, pharmaceuticals that specifically
interact with human receptors or enzymes, and therefore result in specific effects on key
biological functions, are classified as potential “HC” (Fig. 5).
Receptors or enzymes that regulate important pathways are often conserved between the
different vertebrate groups. Moreover homologies between vertebrate and invertebrate
receptors and enzymes are often surprisingly high. In general, homologies of receptors in
vertebrates (fish, amphibians, reptiles, birds) are higher than in invertebrates or plants, which
is reasonable on an evolutionary basis.
The so far known sequences homologies for
receptors and enzymes in fish compared to humans may reach over 90%, in particular for
nuclear hormone receptors (e.g. rainbow trout mineralocorticoid receptor (Prunet et al,
2006)). This similarity makes fish, amphibians, birds and reptiles potentially susceptible to
similar biochemical and physiological mechanisms of activation/inactivation as are mammals.
A summary of homologies between human and fish for several important drug targets is
given in Table 7. These data were obtained by the use of the database www.pubmed.gov
and the function Homologene. This allowed first to search for human receptors, and in a
second step, to display alignments with known, sequenced fish orthologues.
Gunnarsson et al predicted orthologes for 1318 human drug targets in 16 species of which
several are relevant for ecotoxicity testing. In this study, zebrafish had orthologs to 85% of
the drug target while only 61% were conserved in Daphnia and 35% in green alga
(Gunnarsson et al., 2008). As a consequence chronic and target organ toxicity identified in
mammalian safety assessments may be suitable to estimate potential toxic effects in fish and
other vertebrates, but not on algae.
55
Data on invertebrates are much more limited and the fact that often similar enzymes and
receptors display different functions in invertebrates than in mammals complicates the issue.
One example for this phenomenon is the enzyme HMG-COA reductase. In invertebrates it
plays an important role in juvenile hormone production. In plants it inhibits a key enzyme for
biosynthesis, but in mammals it mediates the formation of cholesterol (for review: Fent et al.,
2006).
Human receptor
Fish species with
homolog sequence
Identity to human
sequence (%)
Estrogen receptor 1 (alpha)
Zebrafish (Danio rerio)
Rainbow-trout (Oncorhynchus
mykiss)
Salmon (Salmo salar)
Killifish (Fundulus heteroclitus)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Rainbow-trout (Oncorhynchus
mykiss)
Japanese Medaka (Oryzias
latipes)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Killifish (Fundulus heteroclitus)
Japanese Medaka (Oryzias
latipes)
Zebrafish (Danio rerio)
Killifish (Fundulus heteroclitus)
Rainbow-trout (Oncorhynchus
mykiss)
Japanese Medaka (Oryzias
latipes)
Zebrafish (Danio rerio)
Zebrafish (Danio rerio)
Rainbow-trout (Oncorhynchus
mykiss)
Japanese Medaka (Oryzias
latipes)
Fathead Minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
Salmon (Salmo salar)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Zebrafish (Danio rerio)
Rainbow trout (Oncorhynchus
mykiss)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Rainbow trout (Oncorhynchus
53/56
57.1/48.3
Estrogen receptor 2 (beta)
Estrogen-related receptor alpha
Estrogen-related receptor
gamma
Progesterone receptor
Androgen receptor
Thyroid hormone receptor alpha
Thyroid hormone receptor beta
Glucocorticoid receptor
Mineralocorticoid receptor
56
56
50.4
58
56.2/68.8
54.8/62.5/58.9
53.8
52.5
67.9
60.5
61.4
64/72.2/86.9/93.6
76.7/71.9/78.2
79.8
81.8/91/71.8/75.3
64
69.3/71.3/73.5/69.3
70.6/68.5
68.5
68.8
88.2/90.2
83.9
84
84.1
89
49
53/52
50
58.8/50
57.6
Retinoic acid receptor RAR
alpha
Retinoic acid receptor RAR beta
Retinoic acid receptor RAR
gamma
Retinoid X receptor alpha
Retinoid X receptor beta
Retinoid X receptor gamma
Peroxisome proliferator­
activated receptor alpha
Peroxisome proliferator­
activated receptor gamma
Peroxisome proliferator­
activated receptor delta
Adrenoreceptor alpha 1A,1B and
1D
Adrenoreceptor beta
GABA A receptor
GABA receptors GABRR1-3,
GABRQ
mykiss)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
Fathead minnow (Pimephales
promelas)
Salmon (Salmo salar)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
Zebrafish (Danio rerio)
Rainbow trout (Oncorhynchus
mykiss)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Rainbow trout (Oncorhynchus
mykiss)
Japanese Medaka (Oryzias
latipes)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
Fathead minnow (Pimephales
promelas)
Salmon (Salmo salar)
Zebrafish (Danio rerio)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Salmon (Salmo salar)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
Fathead minnow (Pimephales
promelas)
Salmon (Salmo salar)
Zebrafish (Danio rerio)
Rainbow trout (Oncorhynchus
mykiss)
Zebrafish (Danio rerio)
Rainbow trout (Oncorhynchus
mykiss)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
Fathead minnow (Pimephales
promelas)
Zebrafish (Danio rerio)
Japanese Medaka (Oryzias
latipes)
57
86/90.4/79.8
89
79.4
87.7
74
89.5
85/90.8
88.5
86/91.1/92.6
89.7
93.2/84.5
76/74/80.5
64.3
88.7/68
77.5
74/76/82
83.5/92.8
61
82.3/82.7/92.9
72/76/74.6
74.3/60.7
70/69.3
61.2
71/79.6
75.4
73.5
77.8/73.6/76.8/74.6
59/58/68/73/60
61/60/65/50
56/52
50
80/84/74/88/90
77.5
78
71/74/76/66/77/78/80/87.5
76
Table 7: Homology between human and fish for several receptors that may serve as drug
targets. (Data from www.pubmed.gov). For some receptors more than one value exists due
to the fact that they were sequenced from different labs independently.
Furthermore, although pharmaceuticals may act on specific receptors, they may have
different effects in invertebrates and algae: Beta-blockers may affect photosynthesis in algae
besides general narcosis-type effects (Escher et al., 2006). It is also intriguing that highly
conserved estrogen receptors are found in vertebrates, but are lacking in mussels. In Mytilus
spp, only estrogen receptor-like proteins have been identified on the mRNA level and it is
currently unknown whether they play a similar function as in vertebrates (Canesi et al.,
2007).
Conclusions
Based on the evolutionary conservation of receptors, an important step to identify “HC”
includes the evaluation of the degree of homology between the human target and the
corresponding receptor/enzyme of aquatic organisms in case where this is known. The
higher the degree of homology, the higher is the risk of the drug to elicit effects in aquatic
organisms at low environmental concentrations. Based on the results displayed in Table 7,
the analysed receptors can be separated into two groups according to the degree of
homology to the human target. The first category consists of receptors having 60% or more
identity, members of this group are at high risk to be affected by the human drug at low
concentrations. The second category includes receptors having around 50% or less
homology. For these receptors the risk to be effected by the human pharmaceutical is lower
(Table 8). This categorization is based on pragmatic considerations.
58
Table 8: Risk of fish receptors according to the degree of homology to the human analogue.
3.1.3 Classification of regulated effects through conserved receptors
The basis to identify “however” compounds, is the classification of the effects related on the
action of the receptor/enzyme. Figure 5 shows a flow scheme to be followed for a certain
compound. Only pharmaceuticals that target receptors/enzymes regulating important
biological pathways and target organs have the potential for being “HC”. Among them are
seven critical target pathways and associated receptors identified in animals. If they are
affected, negative implications for the maintenance of healthy populations are possible.
Namely, targets are those regulating cell division or those that have developmental,
hormonal, reproductive, immunological and neurological effects. For plants including algae,
adverse effects on photosynthesis are most important.
One important step of the mode of action concept is the identification of homologous and
conserved targets (receptors or enzymes) of the drugs in non-target organisms. As not all
cellular pathways targeted by the pharmaceutical regulate important or essential processes
for health and survival, a classification of critical conserved receptors or target biomolecular
structures is needed. By applying a ranking of targets it is possible to identify receptors
according to their importance. Drugs targeting critical pathways and important receptors and
cellular events have a higher risk to be a “HC” than drugs acting unspecifically or interact
with drug metabolizing enzymes, for instance.
Hence, we propose the following classification, which represents a pragmatic concept for use
in regulation practice. We propose a hierarchical approach addressing the most important
biological targets first with decreasing order of importance for health and survival. We
assume that by following this pragmatic (and somewhat arbitrary) hierarchy, potentially
59
harmful compounds can better be identified that regarding all targets similarly at same weight
and importance. For pragmatic and biological reasons, two main categories having different
consequences are proposed:
1. All receptors/enzymes involved in the regulation of reproduction and development.
Also included are all the receptors/enzymes regulating cell proliferation (including cell
signalling), the nervous, endocrine and immune system and photysynthesis. Included
are also inhibitors of key enzymes in hormone biosynthesis (e.g. inhibitors of
aromatase)
2. Receptors/enzymes involved in the regulation of metabolism.
Figure 5: Flow scheme and criteria for the identification of “however” compounds (HC)
among human pharmaceuticals in a stepwise mode of action concept.
In the first category, potential “HC” are found that affect critical targets and that may excert
their adverse effects at low concentrations. In the second category, effect concentrations
60
may be higher as targets are not essential or very critical for the survival of the organism or
the maintenance of a population. For such compounds additional ecotoxicological tests are
necessary.
It has to be taken into account that this classification holds true for animals. For algae and
plants it may look differently, because for these organisms photosynthetic production,
growth, reproduction and survival are the most important processes. Therefore drugs
targeting receptors/enzymes that are involved in humans and mammals in regulating specific
effects may also unintentionally be involved in the regulation of photosynthetic production,
growth and survival. A case in point are the HMG-CoA reductases, which are key enzymes
for biosynthesis that have the highest risk to be “however” compounds for algae and plants
(Fent et al., 2006).
Important aspects in the application of the mode of action concept
Knowledge of the modes of action in mammals is not a complete and reliable means of
predicting effects in other organisms. This holds true in particular for organisms having larger
biological distance to mammals such as invertebrates and plants. Receptors may be
conserved in these species but their specific function, and hence, the effects of
pharmaceuticals may differ from those in mammals and/or vertebrates. To date, not much is
known on the evolutionary conservation of receptors (with the exeption of a few vertebrate
species, enzymes and other biomolecules, which are important as targets of pharmaceuticals
(for review Fent et al., 2006). Therefore, further investigations are necessary to elucidate the
role of conserved mammalian or vertebrate receptors in invertebrates, similar to the work by
Gunnarsson et al. 2008.
Further research is also required to extrapolate the modes of action concept to invertebrate
species. There are several important points to be clarified before a reasonable extrapolation
to the mechanisms of actions to invertebrate species or even plants is possible: The function
of conserved receptors in non-target species has to be investigated, and additional effects,
which cannot be predicted from the current approach, have to be identified by pertinent
toxicological experiments. Furthermore, the extrapolation to algae and plants should be
considered. In this case, negative effects on photosynthesis are critical in addition to cell
proliferation, development and reproduction, which are key targets in animals too.
We propose that this mechanism-based concept outlined in Figure 5, should essentially be
paralleled and complemented with additional concepts, as for many drugs the modes of
actions are unknown. Therefore we propose to include additional, complementing
approaches such as the fish plasma model.
61
3.1.4 Application of the mode of action concept for selected pharmaceuticals
A few pharmaceuticals were selected for analysis using the mode of action concept for their
potential to be “HC” (Table 9). For these substances, some information on ecotoxicological
effects does exist. The group of analyzed pharmaceuticals includes the known “HCs” EE2
and levonorgestrel, important pharmaceuticals as suspected “HC” and the non “HC”
pharmaceutical paracetamol. The three main questions of the concept were answered by
using
the
following
sources:
www.drugbank.ca,
www.pubmed.gov
and
own
categorization/priorisation of signalling pathways according to Figure 5.
The following three questions are answered:
1. Specific mode of action: yes or no?
2. How strong is the homology between human drug target and fish homologe?
3. How important is the regulated pathway?
Compound
Mode of action concept
Outcome
Ethinylestradiol 1. Mode of action: specific binding to estrogen receptor
(EE2)
2. Homology human target versus fish receptor: up to
90%
> high
3. Receptor function: reproduction
> regulates important pathways for fertility and
reproduction
Levonorgestrel 1. Specific binding to estrogen and progesterone
receptor
2. Homology from 64 up to 90% > high
3. Receptor function: reproduction
> regulates important pathways for fertility and
reproduction
Tamoxifen
1. Specific binding to estrogen receptor
2. Homology up to 90% > high
3. Receptor function: reproduction
> regulates important signalling pathways for fertility
and reproduction
Propranolol
1. Specific binding to adrenoreceptor
2. Homology: 50-60% > high
3. Receptor function: effects at heart rate, cardiac
output, blood pressure, muscle relaxation
> regulated pathways minor important
Fluoxetine
1. Specific: SSRI
2. Homology: over 70% > high
3. Functions: effects at CNS, effects on behaviour
> regulation of important pathways
Paracetamol
1. Specific inhibition of Cox-enzymes
2. Homology: around 60%: high
3. Function: effects on muscles, neurons, platelets and
cell growth > pathways of minor importance
Diclofenac
1. Specific inhibition of Cox-enzymes
62
High risk to be
HC
High risk to be
HC
High risk to be
HC
Low risk to be
HC
Moderate to high
risk to be HC
Low risk to be
HC
Low risk to be
Diazepam
2. Homology around 60%: high
3. Function: effects on muscles, neurons, platelets and
cell growth > pathways of minor importance
1. Specific binding to GABA receptors
2. Homology: 80-90% > high
3. Function: effects on neuronal activity, sedative
properties, muscle relaxant, anticonvulsant > functions
of minor importance
HC
Low risk to be
HC
Table 9: Analysis of pharmaceuticals using the mode of action concept for their potential to
be “HC”.
The key criteria for the identification of potential “HC” are:
- Mode of Action: the more specific the mode of action of a drug is the higher is the risk of the
drug to elicit unwanted effects in aquatic organisms at low concentrations.
- Degree of homology between the human drug target and the potential target in the aquatic
organism. A high amount of sequence homology increases the risk of unwanted effects in
aquatic organisms.
- Importance of effected pathway. If the drug affects an important pathway like reproduction
than the risk of the drug to be “HC” is high.
The obtained results by using the mode of action concept are in accordance to already
known data. For example, our concept classified EE2 and levonorgestrel, the already known
“HCs”as high risk compounds. This also holds true for tamoxifen and fluoxetine. However, for
these two compounds, the NOEC lies over 10 ng/L. In contrast, paracetamol was also
identified by our concept as a compound with low potential to be “HC”. So far our concept
seems to be useful to identify “HC”. But further analysis, with a lager set of pharmaceuticals,
considering PEC and NOEC, has to be performed.
3.2 The fish plasma model concept – an additional/complementary
model to identify “HC”
For the risk assessment of pharmaceuticals, it would be helpful to know the fish plasma
concentration of pharmaceuticals additionally to the PEC. By comparing the fish plasma
concentration with the human therapeutic plasma concentration respectively with toxic
plasma concentration obtained during the toxicological tests, the potential risk of the
pharmaceutical for the fish could be estimated. But so far, data about fish plasma
concentrations are rare. But there exists a theoretical approach developed by Huggett to
calculate the fish plasma concentration based on environmental concentrations and water
solubility of the pharmaceutical. The fish plasma model (Huggett et al., 2003) estimates fish
63
plasma concentrations of drugs, which are used to define the need for further investigations
of chronic effects in fish according to calculated values. The model can be summarized as
follows: In case the estimated fish plasma concentration of a pharmaceutical is close to its
human therapeutic plasma concentration, the compound is assumed to have potential
adverse effects in aquatic organisms.
The model uses the information about concentrations of drugs in human plasma. Moreover it
is based on the hypothesis that the main targets of the drug (receptors and enzymes) are
similar between humans and fish, and so are the many physiological processes. In the fish­
plasma model the extensive scientific and mechanistic understanding of pharmaceuticals in
mammalian systems is used for the selection of pharmaceuticals with potential (chronic) risks
for aquatic organisms. The main concept behind the model is the assumption of the similarity
of the (wanted) plasma concentration of the drugs in humans and the (unwanted) effect
concentrations in fish. Therefore pharmaceuticals with the highest potential for chronic
effects are those for which the predicted plasma concentration in fish is close to the plasma
concentration in humans.
3.2.1 Description of the model
As mentioned, the key assumption for the fish-plasma model is the similarity of many
receptors and enzymes across mammalian and even non-mammalian species. The best
model would be based on the highest human plasma steady state concentration (drug effect
concentration) that corresponds to a NOEC (HSSPC NOEC). This concentration could than
be compared to a measured fish steady state plasma concentration (FSSPC). The ratio of
these two plasma concentrations could than be expressed as a margin of safety:
Margin of Safety = HSSPC NOEC/FSSPC
This margin of safety should be as large as possible for human pharmaceuticals. But
unfortunately on one side drug levels in humans are reported as the recommended dose and
not as the NOEC. On the other side, there are very limited data on steady state fish plasma
concentrations for pharmaceuticals. As an alternative to this approach, an effect ratio (ER)
can be calculated. As the human therapeutic plasma concentration HTPC at the maximum
dose of a drug is available it can be used instead of the HSSPC NOEC. The fish plasma level
FSSPC can be calculated by using the predicted or measured environmental concentration of
a certain drug and its Log KOW. These two values can be applied to calculate the ER:
ER = HTPC/FSSPC
64
As with the margin of safety, the ER ratio should be as large as possible. If the ER is lower or
equal to 1, than there is a potential for receptor mediated responses in fish. The lower the ER
is, the greater the need for additional chronic investigations in fish.
HTPCs are investigated as a standard value during the drug development process. They can
be expressed as a single point in time (Cmax) or as a function of time (AUC). Both values
can be obtained from databases (for example: Online Physicians Desk References
(www.pdr.net)).
FSSPC can be estimated by using the Log KOW value and a measured or predicted
environmental concentration. Accumulation via food chain is not considered in this
assumption. The partitioning of a substance between the aqueous phase and the arterial
blood in trout is estimated by Fitzsimmons (Fitzsimmons et al, 2001):
Log PBlood:Water = 0.73*Log KOW-0.88
Using these relationships, drug partitioning between blood and water for a given
environmental concentration can be equated to a fish steady state plasma concentration:
FSSPC = EC*(PBlood:Water)
The major driving force in this model for a compound crossing from water into the blood of
fish is the hydrophobicity. It should be noted that in this model metabolism, excretion or
protein binding of the drug in fish is not taken into account. Therefore the model can be
regarded as a worst case scenario in that it is assuming maximal blood levels. In addition,
calculation of bioconcentration factors based on the sugggested formula using the Kow is not
validated for ionizable compounds like pharmaceuticals.
Huggett suggested the use of safety factors to refine the degree of uncertainty expressed in
the model. As an initial approach, a safety factor of 1000 is reasonable. This factor is derived
by applying a 10-fold factor for extrapolation of humans to animals, a 10-fold factor for
sensitivity differences and a 10-fold factor for extrapolating from mammalian to non­
mammalian species. Therefore, compounds with an effect ratio lower than 1000 are
regarded as critical for potential adverse effects, needing additional assessment in fish.
65
3.2.2 Application for selected pharmaceuticals
To evaluate the fish plasma model, we calculated the fish plasma concentration (Table 10)
and the effect ratio, without safety factors (Table 11) applying the mathematical formula of
Huggett (Huggett et al., 2003) for some pharmaceuticals including the known “HCs” EE2 and
levonorgestrel. We did not apply safety factors (or application factors) to keep the analysis as
simple as possible. By this analysis we wanted to get a first idea whether or not the fish
plasma model might be a useful model for such purposes. We did not want to conduct a
datailled analysis of the fish plasma model. For levonorgestrol, no information about
environmetal concentrations is available. Therefore we used 10 ng/L (according to the EMEA
guideline) and 100 ng/L as environmental concentrations to calculate the fish plasma model.
Considering estimated fish plasma concentrations of certain pharmaceuticals, we calculated
the effect ratio for the pharmaceuticals according to (Huggett et al., 2003) (Table 11).
Compounds with an effect ratio below 1 have a potential risk to be effective in fish resulting in
adverse effects. Fig. 6 shows the correlation between human and fish plasma concentration.
The following critical drugs for which fish plasma concentrations are close to human plasma
concentrations or even above human plasma concentrations, were identified: Propranolol,
tamoxifen, levonorgestrel and EE2. For three additional compounds, diclofenac, metoprolol
and fluoxetine, fish plasma concentrations are close to human plasma concentrations and
must therefore be considered as potential critical drugs. For all other drugs estimated fish
plasma concentrations are much below the human plasma concentrations and can therefore
be considered as compounds not relevant for the discussion of effects at low concentrations.
Pharmaceutical
Log
KOW
Log
Pblood:water
Pblood:water
Acetylsalicylic acid
Salicylic acid
Dextropropoxyphene
Diclofenac
Fenoprofen
Ibuprofen
Indometacin
Ketoprofen
Mefenamic acid
Naproxen
Paracetamol
Phenazone
Acebutolol
Metoprolol
Nadolol
1.19
2.26
4.90
4.22
3.64
3.97
3.66
3.56
4.04
3.31
0.46
-0.0113
0.7698
2.697
2.2
1.77
2
1.8
1.72
2.1
1.5
-0.5
0.97
5.89
497.7
158
58.9
100
63.1
52.5
126
31.6
0.32
2.36
2.48
1.40
0.84
0.93
0.14
6.9
8.5
1.38
66
Environ
mental
conc.
(ng/L)
500
5000
FSSPC
(µ
µg/ml)
HTPC
(µ
µg/ml)
4.85*10-5
0.029
1100
200
6000
400
200
500
800
10000
1000
0.173
0.012
0.6
0.025
0.011
0.063
0.0252
3.2*10-3
1400
0.012
20-200
20-200
0.05-0.3
0.5-3
30-60
15-30
0.3-1
1-6
2-10
20-50
10-25
5-25
0.2-2
0.035-0.5
0.01-0.25
Oxprenolol
Propranolol
Bezafibrate
Clofibric acid
Fenofibrat
Gemfibrazol
Carbamazepine
Diazepam
Fluoxetine
Cyclophsophamide
Ifosfamide
Tamoxifen
Albuterol
Caffeine
Ethinylestradiol EE2
Estradiol
Levonorgestrel
3.59
1.8
63.1
3.58
5.58
4.77
2.45
2.82
5.37
1.7
3.2
2.60
0.91
1.18
3.04
50.1
1584
398.11
8.1
15.13
1096.5
-0.01
6.59
1.31
-0.07
3.63
3.57
3.8
-0.89
3.9
0.08
-0.93
1.77
1.73
1.894
0.13
7943
1.2
0.12
58.9
53.7
78.34
600
1400
700
0.038
900
1000
5
60
0.358
8.1*10-3
7.5*10-5
0.066
400
50
6000
0.5
0.5
10
100
31.77
6*10-5
7.2*10-4
2.9*10-5
2.6*10-5
7.834*10-4
7.834*10-3
0.05-0.3
0.02-0.3
-15
0.035
5-30
0.250
2-8
0.2-2
0.16-0.5
10-25
0.05-0.5
0.01-0.02
4-10
0.00001
0.002­
0.02
Table 10: Application of the fish plasma model for estimation of the fish plasma
concentrations of various pharmaceuticals with known environmental concentrations. Log
Pblood:water and FSSPC were calculated according to the fish-plasma model (Huggett et al,
2003). Values for environmental concentrations were taken from literature (Fent et al., 2006),
log KOW values were taken www.drugbank.ca., and information about the human therapeutic
plasma concentrations from medical internet sources www.drugs.com, www.drugbank.ca
and www.drugsafetysite.com.
Effect ratio
(HPC/FPC)
Acetylsalicyclic acid
Human plasma
concentration
(HPC, µg/mL)
20
Estimated fish
plasma conc. (FPC,
µg/mL
0.0000485
41237
Salicyclic acid
20
0.029
690
Diclofenac
0.5
0.173
3
Fenoprofen
30
0.012
2500
Ibuprofen
15
0.6
25
Indometacin
0.3
0.025
12
Ketoprofen
1
0.011
91
Mefenamic acid
2
0.063
32
Naproxen
20
0.0252
794
Paracetamol
10
0.0032
3125
Metoprolol
0.035
0.012
3
Propranolol
0.02
0.038
0.5
Carbamazepine
2
0.0081
247
Pharmaceutical
67
Diazepam
0.2
0.000075
2667
Fluoxetine
0.16
0.066
2.4
Tamoxifen
0.05
31.77
0.002
Albuterol
0.01
0.00006
167
Caffeine
4
0.00072
5556
Ethinylestradiol EE2
0.00001
0.000026
0.4
Levonorgestrel
0.002
0.00078
2.56
0.0078
0.256
Table 11: Human plasma concentrations (HPC), estimated fish plasma concentration (FPC)
and effect ratios for estimation of potential effects. To calculate the effect ratio according to
fish plasma model (Huggett et al., 2003) the minimal effect concentration for the HPC was
used to analyse for a worst case scenario.
Figure 6: Correlation between the estimated fish plasma concentration (FPC) and human
therapeutic plasma concentrations (HPC) of selected pharmaceuticals. All dots in the area
above the line (1: EE2, 2: levonorgestrel, 3: propranolol, 4: tamoxifen) are compounds having
a potential risk for fish. Squares which are near the line but below (5: metoprolol, 6:
fluoxetine, 7: diclofenac) are compounds close to the cut off of 1. These drugs may have the
potential for adverse effects in fish. The other triangel refer to pharmaceuticals with lower
potential to display a risk for fish. But still, a risk for fish can not be excluded. Due to
graphical reasons, FPC and HPC were converted to logarithmic values.
68
3.2.3 Comparison of fish plasma model and mode of action concept
To further evaluate the usefulness of the developed/described models, we compared the
outcome of the mode of action concept with the outcome of the fish plasma model (Table
12).
Compound
Ethinylestradiol
Levonorgestrel
Tamoxifen
Fluoxetine
Diclofenac
Propranolol
Diazepam
Paracetamol
Fish plasma model Mode of action
HC
HC
HC
HC
HC
HC
-
HC
HC
HC
HC
-
Table 12: Comparison of the results of the two different models.
Table 12 shows that the outcome of the analysis for the pharmaceuticals applying the two
different models is not identical. The fish plasma model identifies diclofenac and propranolol
as potential compounds of potential risk to the environment, which were not identified by
using the mode of action concept. The reason for the classification of these two compounds
as high risk compound by the fish plasma model is due to the high environmental
concentration of both compounds (diclofenac up to 1100 ng/L and propranolol up to 600
ng/L). For levonorgestrel, only the higher environmental concentration (100 ng/L) gave an
effect ratio below 1, but the lower environmental concentration (10 ng/L) gave an effect ratio
above 1. The mode of action concept classified levonorgestrel as potential “HC”. This
outcome is in accordance to experimental data that showed effects of levonorgestrel on fish
at concentrations below 10 ng/L (Zeillinger et al., in 2009). These examples demonstrate that
the outcome of fish plasma model depends on environmental concentrations of a compound.
In this point, the mode of action concept is more conservative and more robust in
identification of potential “HC” because it is based on a mechanistic point of view and the
environmental concentration is not taken into account. However, PECs for the two identified
compounds, diclofenac and propranolol, are above the action limit of 10 ng/L and therefore
experimental fate and effect testing has to be performed anyway.
69
3.4 The QSAR concept
3.4.1 Quantitative Structure-Activity Relationships QSAR
More than a century ago, Crum-Brown and Fraser expressed the idea that the physiological
action of a substance was a function of its chemical composition and constitution (CrumBrown, 1868). Some 40 years ago, Hansch began quantifying relationships between a
compound s physicochemical properties and its biological activity (Fujita et al., 1964). His
series of papers laid the foundations for a concept later referred to as quantitative structure­
activity relationships (QSAR). Since then, researchers have widely employed QSAR, for
example, to predict the activity of new drugs or the toxicity of environmental chemicals. The
aim of QSAR is to predict properties of molecules or classify molecules based on structural
features. Properties can be physical properties like boiling point or aqueous solubility or
biological activities like carcinogenicity or LD50. To perform QSAR analysis, a large group of
descriptors (topological, geometric and electronic properties of the molecule) and known
biological activities/physical properties for a molecule are needed. With the help of
mathematical calculations, correlations between molecule structure and activities/properties
are modelled. This allows predictions about biological activities or physical properties of a
new molecule just by comparing the descriptors of this unknown molecule with the
descriptors of the already known and analysed compounds. An illustration of the model is
shown in Figure 7.
Descriptors of
known/unknown
molecules
Mathematical
Modelling
Biological activity, physical properties
Figure 7: Schematic summary of the QSAR model.
70
3.4.2 Analysis of binding affinities of chemicals or pharmaceuticals to specific
human receptors using VirtualToxLab
In silico techniques for the prediction of toxicological endpoints are extremely appealing
because of their inexpensiveness and the short processing time to generate a big amount of
data. Numerous commercially available and free web-based programs for toxicity prediction
are available; some of them are listed and briefly described below (Muster et al., 2008).
- Classical QSAR approaches: Correlate structural or property descriptors of compounds with
biological activities, QSARs for various endpoints published
- ToxScope: ToxScope correlates toxicity information with structural features of chemical
libraries, and creates a data mining system
- COMPACT: COMPACT is a procedure for the rapid identification of potential
carcinogenicity or toxicities mediated by CYP450s
- MDL QSAR: QSAR modeling system to establish structure-property relationships, create
new calculators and generate new compound libraries
- PASS: comparison of new structures with structures of well-known biological activity
profiles by using structure descriptors
- MetaDrug: Assessment of toxicity by generating networks around proteins and genes
(toxicogenomics platform)
- CADD: Computer-aided drug design (CADD) by multi-dimensional QSARs applied to
toxicity-relevant targets
- PreADMET: Calculation of important descriptors and neural network for the construction of
prediction system
- Admensa Interactive: QSAR-based system primarily for ADME (absorption, distribution,
metabolism and excretion) optimization
- BfR decision support system: Rule-based system using physicochemical properties and
substructures
The VirtualToxLab, University of Basel (www.biograf.ch), is also an in silico tool for predicting
the toxic potential of existing and hypothetical compounds (drugs and environmental
71
chemicals) by simulating and quantifying their interactions with human receptors at the
molecular level using automated multi-dimensional QSAR. Currently, it includes 11 validated
models for the aryl hydrocarbon, estrogen α/β, androgen, thyroid α/β, glucocorticoid,
mineralocorticoid and peroxisome proliferator-activated receptor γ as well as for the enzymes
CYP450 3A4 and CYP450 2A13. The models were trained using a representative selection
of 667 substances and validated with 207 compounds different there from. This model could
be useful for the identification of potential “HC”. As it predicts the affinity respectively the
potential binding of pharmaceuticals to human receptors, it could be a good additional model
beside the mode of action concept.
3.4.3 Dimensionality of QSAR approaches
The dimensionality of QSAR analysis describes how many parameters are used to calculate
respectively to model the binding affinity of a compound to a selected receptor. The first
QSAR analysis (1D-QSAR) used only a few parameters like logP, hydrophobicity, electronic
properties and steric parameters (Vedani et al., 2006). The most recently developed QSAR
today, is the 6D-QSAR. There beside the use of the 5D-QSAR, different solvation scenarios
are taken into account (Vedani et al., 2006). A detailed description of all existing QSAR
levels is shown in Table 13.
Dimension Method
1D-QSAR
Affinity correlates with pKa, logP, electronic properties etc.
2D-QSAR
Affinity correlates with structural patterns
3D-QSAR
Affinity correlates with three-dimensional structure of the ligands
4D-QSAR
Ligands are represented as an ensemble of conformers, orientations,
protonation states and stereoisomeres
5D-QSAR
Like 4D, additionally representation of different induced-fit models
6D-QSAR
Like 5D, additionally representation of different solvation scenarios
Table 13: Different levels of QSAR analysis according to the used parameters (Vedani et al.,
2006). The induced-fit is the change in the binding pocket of the receptor induced by the
ligand to facilitate the binding between receptor and ligand. The induced-fit represents a local
phenomenon including for example the subtle rearrangement of few side chains. It may not
72
only alter the topology of the binding pocket but the character (hydrophobic/hydrophilic,
dielectric properties) of subpockets or the solvent accessibility of the binding site.
By using these models the “toxic potential” could be calculated. The “toxic potential” of the
analysed compounds is expressed as weighted toxic potential (WTP). It derives from the
calculated binding affinities towards the 11 target proteins presently comprised in the
VirtualToxLab. The WTP as derived from the VirtualToxLab is based solely on
thermodynamic considerations. It does not include any toxicokinetic processes such as
adsorption, distribution, metabolism, elimination or target tissues. It is assumed that the only
important parameter to predict toxicity of a compound is its binding affinity to the analysed
receptor (thermodynamic analysis). The WTP ranges from 0.0 (none) to 1.0 (extreme) and
has arbitrarily been split into five classes: WTP > 0.85 (****, extreme risk), WTP > 0.675 (***,
high risk), WTP > 0.5 (**, risk), WTP > 0.325 (*, low risk) and WTP < 0.325 (–, no risk).
These classes are solely directed to humans and mammals. Table 14 gives examples of
pharmaceuticals belonging to different categories with yielded WTP. High numbers refer to
probable interactions with important receptors. The higher the WTP, the more probable is the
interaction. The high potential to interact with hormone receptors means that these
pharmaceuticals are probable candidates for being “HC” compounds.
Compound
Weighted Toxic
WTP
Weighted main
Potential
Class
target(s)
Bicalutamide
0.678
***
→ GR, ERβ
Budesonide
0.722
***
→ GR, ERβ
Ezetimibe
0.836
***
→ GR, ERα, ERβ
Fluticasone
0.777
***
→ GR, ERα, ERβ, AR
Diethylstilbestrol
0.827
***
→ ERα, ERβ
Bendroflumethiazide (R/S)
0.690/ 0.540
***
→ GR, ERα
Dehydroepiandrosterone
0.792
***
→ ERα, ERβ, AR
Finasetride
0.852
***
→ GR, ERα, ERβ, AR
Prednisone
0.704
***
→ ERβ, AR
WTP ***
73
Spironolactone
0.788
***
→GR, ERα, AR
Ulobetasol
0.720
***
→ GR, ERβ, AR, AhR
Ethinylestradiol
0.834
***
→ ERα, ERβ, AR
Levonorgestrel
0.801
***
→ ERα, AR
Tamoxifen
0.621
**
→ GR, AR
Fluoxetine (R/S)
0.519/ 0.401
**/*
→ GR, AR
Diclofenac
0.131
-
Propranolol (R/S)
0.344/ 0.397
*/*
→ GR, AR
Paracetamol
0.328
-
→ AR, ERβ
Drugs used in the
analysis
Table 14: The “toxic potential” of compounds calculated using multi-dimensional QSAR. The
more asterisks are, the higher the toxic potential of the different compounds is, according to
the Virtual ToxLab program (www.biograf.ch). Weighted main targets are estrogenreceptor
α,β (ERα,β), androgenreceptor (AR), thyroid receptor α,β (TRα,β) and aryl hydrocarbon
receptor (AhR).
Most recently, an Internet Portal to the VirtualToxLab was developed (www.biograf.ch). This
allows easy access to the technology. Upon uploading the 3D coordinates of a compound of
interest, the compound will be automatically processed and tested against the selected
virtual test kits. Whether this tool is actually applicable to ecotoxicology and other animals
than mammals has to be evaluated by further investigations. Moreover it must be evaluated
whether the lack of toxicokinetics in this model hampers its usefulness. This new tool is,
however, a useful contribution as a first screening tool if no other data are available. In case
compounds ingestions are identified to interact strongly with important hormone receptor,
they may be regarded as potential “HC”.
3.4.4 ECOSAR
ECOSAR (Ecological Structure Activity Relationships) is a computer software program that is
used to estimate the toxicity of chemicals used in industry and discharged into water (U.S.
environmental protection agency). The program predicts the toxicity of industrial chemicals to
74
aquatic organisms such as fish, invertebrates, and algae by using Structure Activity
Relationships (SARs). SAR, is a technique routinely used to estimate aquatic toxicity of
chemicals. The program estimates a chemical's acute (short-term) toxicity and, when
available, chronic (long-term or delayed) toxicity. ECOSAR uses SARs to predict the aquatic
toxicity of chemicals based on their structural similarity to chemicals for which aquatic toxicity
data are available. SARs express the (assumed) correlations between a compound's
physicochemical properties and its aquatic toxicity. SARs measured for one compound can
be used to predict the toxicity of similar compounds belonging to the same chemical class.
ECOSAR also allows access to over 100 SARs developed for 42 chemical classes, not
including pharmaceuticals. The SARs contained within the program are based on test data.
Many of the SAR predictions have been validated. Most SAR calculations in the ECOSAR
programs are based upon the octanol/water partition coefficient (KOW). ECOSAR is used by
the U.S. Environmental Protection Agency (EPA) to predict the aquatic toxicity of new
industrial chemicals in the absence of test data. The use of SARs in the U.S.A. is a popular
practice for estimating ecotoxicity for many chemicals. A disadvantage of such models based
on physicochemical properties is the occurrence of compounds that have a specific mode of
action. In these cases, false results may result. As the predictions are based on the KOW, the
risk of compounds with a specific mode of action and therefore with the potential to elicit
unwanted effects at low concentrations, but a low KOW could be underestimated. Additionally,
the specific interaction of such compounds with receptors and key biological processes is not
modelled in ECOSAR. ECOSAR is thus considered not suitable for evaluation of effects of
pharmaceuticals at low concentrations.
3.5 Comparison of fish plasma model, mode of action concept and
VirtualTox lab
To further evaluate the usefulness of the developed/described models, we compared the
outcome of the three models. The mode of action concept is compared to the fish plasma
model and the VirtualTox lab model (Table 15).
Table 15 shows that the outcome of the analysis for the pharmaceuticals applying the three
different models is not identical. The fish plasma model identifies diclofenac and propranolol
as potential “HC”, which were not identified by using the mode of action concept and the
VirtualTox lab. The reason for the classification of these two compounds as high risk
compound by the fish plasma model is due to the high environmental concentration of both
compounds (diclofenac up to 1100 ng/L and propranolol up to 600 ng/L).
75
Compound
Fish plasma model Mode of action concept VirtualTox lab
Ethinylestradiol
Levonorgestrel
Tamoxifen
Fluoxetine
Diclofenac
Propranolol
Paracetamol
HC
HC
HC
HC
HC
HC
-
HC
HC
HC
HC
-
High Risk (WTP:0.834)
High Risk (WTP:0.801)
Risk (WTP: 0.621)
Risk (WTP: 0.519)
No risk (WTP: 0.131)
Low risk (WTP: 0.397)
No risk (WTP: 0.328)
Table 15: Comparison of the results of the three different models.
These examples demonstrate that the outcome of fish plasma model depends on
environmental concentrations of a compound. In this point, the mode of action concept and
the VirtualTox lab are more conservative and more robust in identifying potential “HC”,
because it is based on a mechanistic point of view and the environmental concentration is
not taken into account. It should be noted that the predicted environmental concentrations of
the two identified compounds, diclofenac and propranolol, are higher than the action limit of
10 ng/L, and therefore, additional testing has to be performed anyway.
3.6 Conclusions
We propose a stepwise approach for the identification of “HC” taking the different concepts
into account. A key role plays the mode of action concept with the three major steps: (1)
identification of the mode of action taking the toxicological information into account; (2)
identification of effected/regulated pathways and (3) classification in minor and major
important pathways. Beside of these three major criteria for the identification of “HC”, we
propose to include the information about target homology and the toxicological data. The
target homology can be helpful to estimate the risk that a compound will display effects in
other species than humans. The toxicological data can be helpful for the identification of
additional (unwanted) target pathways of the pharmaceutical that than also have to be
included in the analysis. Additionally, we propose to include the VirtualToxLab model to
support the mode of action concept and for the identification of additional targets of the
pharmaceutical. The fish plasma model is taken into consideration in addition for prediction
of the fish plasma concentration of a pharmaceutical for comparision with the human
therapeutic plasma concentration, and therefore to complement the identification. However,
is should be emphasised that the estimated fish plasma concentration depends largely on
76
the compound´s KOW and on the estimated environmental concentration and it is thus
considered less useful. A summary of our proposed combined approach is shown in Figure
8.
Following the evaluation of the proposed approach on model compounds, the applicability of
the mode of action concept and the other concepts discussed needs to be tested by
regulators, contract laboratories, and the industry during environmental risk assessments.
Further discussion on which approach or combination of approaches are best suited to
identify all substances with effects at low concentrations is needed.
Fig. 8: Summary of our proposed approach for the identification of “HC”.
4. Environmental risk assessment: Phase II: environmental
fate and effects analysis
For highly active substances (HC), the EMEA guideline recommends a tailored risk
assessment stategy that addresses the specific mechanism of action of the compound. The
project also aimed at addressing the question whether effects at low concentrations would be
detected by the standard tests required in Tier A or if an alternative testing strategy is
necessary.
77
Phase II, Tier A of the EMEA guideline requires the activated sludge, respiration inhibition
tests (OECD 209), the algae growth inhibition test (OECD 201), the Daphnia sp. reproduction
test (OECD 211), and the fish early life stage toxicity tests (OECD 210) for the aquatic
compartment.
OECD 201
Alga, Growth Inhibition Test
The purpose of this test is to determine the effects of a substance on the growth of
freshwater microalgae and/or cyanobacteria. Exponentially growing test organisms are
exposed to the test substance in batch cultures over a period of normally 72 hours. In spite of
the relatively brief test duration, effects over several generations can be assessed. The
system response is the reduction of growth in a series of algal cultures (test units) exposed
to various concentrations of a test substance. The response is evaluated as a function of the
exposure concentration in comparison with the average growth of replicate, unexposed
control cultures. For full expression of the system response to toxic effects (optimal
sensitivity), the cultures are allowed unrestricted exponential growth under nutrient sufficient
conditions and continuous light for a sufficient period of time to measure reduction of the
specific growth rate. Growth and growth inhibition are quantified from measurements of the
algal biomass as a function of time. Algal biomass is defined as the dry weight per volume,
e.g. mg algae/litre test solution. The test endpoint is inhibition of growth, expressed as the
logarithmic increase in biomass (average specific growth rate) during the exposure period.
From the average specific growth rates recorded in a series of test solutions, the
concentration bringing about a specified x % inhibition of growth rate (e.g. 50%) is
determined and expressed as the ErCx (e.g. ErC50). In addition, the lowest observed effect
concentration (LOEC) and the no observed effect concentration (NOEC) may be statistically
determined. Several species of non-attached microalgae and cyanobacteria may be used:
Green
algae:
Pseudokirchneriella
subcapitata,
(formerly
known
as
Selenastrum
capricornutum), ATCC 22662, CCAP 278/4, 61.81 SAG; Desmodesmus subspicatus
(formerly known as Scenedesmus subspicatus) 86.81 SAG
Diatoms: Navicula pelliculosa, UTEX 664;
Cyanobacteria: Anabaena flos-aquae, UTEX 1444, ATCC 29413, CCAP 1403/13A;
Synechococcus leopoliensis, UTEX 625, CCAP 1405/1
78
OECD 210
Fish, Early-Life Stage Toxicity Test
Tests with the early-life stages of fish are intended to define the lethal and sub-lethal effects
of chemicals on the stages and species tested.
Principle of the test: The early-life stages of fish are exposed to a range of concentrations
of the test substance dissolved in water, preferably under flow-through conditions, or where
appropriate, semi-static conditions. The test is begun by placing fertilised eggs in the test
chambers and is continued at least until all the control fish are free-feeding. Test duration will
depend upon the species used, examples for different species are shown in Table 18. Lethal
and sub-lethal effects are assessed and compared with control values to determine the
lowest observed effect concentration and hence the no observed effect concentration. The
number of fertilised eggs at the start of the test should be sufficient to meet statistical
requirements.
Species
Recommended test duration
Oncorhynchus mykiss
Pimephales promelas
Danio rerio
Oryzias latipes
2 weeks after control are free-feeding or 60 days post-hatch
32 days from start of the test or 28 days post-hatch
30 days post-hatch
30 days post-hatch
Table 18: Recommended durations of the fish early stage test for different fish species.
Test endpoints: Abnormal appearance: the number of larvae or fish showing abnormality of
body form should be recorded at adequate intervals depending on the duration of the test
and the nature of the abnormality described. It should be noted that abnormal embryos and
larvae occur naturally and can be of the order of several percent in the control(s) in some
species.
Abnormal behaviour: abnormalities, e.g. hyperventilation, unco-ordinated swimming, atypical
quiescence and atypical feeding behaviour should be recorded at adequate intervals
depending on the duration of the test.
Weight: at the end of the test all surviving fish must be weighed.
Length: at the end of the test, measurement of individual lengths is recommended.
These observations will result in the following data being available for statistical analysis:
- Cumulative mortality;
79
- Numbers of healthy fish at end of test;
- Time to start of hatching and end of hatching;
- Numbers of larvae hatching each day;
- Length and weight of surviving animals;
- Numbers of deformed larvae;
- Numbers of fish exhibiting abnormal behaviour.
Recommended species are among freshwater fish: Oncorhynchus mykiss, Pimephales
promelas, Danio rerio, Oryzias latipes
Saltwater fish:
Cyprinodon variegatus
OECD 211
Daphnia magna Reproduction Test
Principle of the test: The primary objective of the test is to assess the effect of chemicals
on the reproductive output of Daphnia magna. To this end, young female Daphnia (the
parent animals), aged less than 24 hours at the start of the test, are exposed to the test
substance added to water at a range of concentrations. The test duration is 21 days. At the
end of the test, the total number of living offspring produced per parent animal alive at the
end of the test is assessed. This means that juveniles produced by adults that die during the
test are excluded from the calculations. The reproductive output of the animals exposed to
the test substance is compared to that of the control(s) in order to determine the lowest
observed effect concentration (LOEC), and hence, the no observed effect concentration
(NOEC). In addition, and as far as possible, the data are analysed using a regression model
in order to estimate the concentration that would cause a x % reduction in reproductive
output, i.e. ECx (e.g. EC50, EC20 or EC10).
OECD 209
Activated Sludge, Respiration Inhibition Test
The method described in this test guideline assesses the effect of a test substance on micro­
organisms by measuring the respiration rate under defined conditions in the presence of
different concentrations of the test substance. The purpose of this test guideline is to provide
a rapid screening method whereby substances which may adversely affect aerobic microbial
treatment plants can be identified and to indicate suitable non-inhibitory concentrations of
test substances to be used in biodegradability tests. The respiration rate is the oxygen
consumption of aerobic sludge or waste-water microorganisms expressed generally as mg
80
per litre per hour. EC 50 in this Test Guideline is the concentration of the test substance at
which the respiration rate is 50% of that shown by the control under conditions described in
this guideline.
Principle of the test: The respiration rate of an activated sludge fed with a standard amount
of synthetic sewage feed is measured after a contact time of 30 minutes or 3 hours, or both.
The respiration rate of the same activated sludge in the presence of various concentrations
of the test substance under otherwise identical conditions is also measured The inhibitory
effect of the test substance at a particular concentration is expressed as a percentage of the
mean respiration rates of two controls. An EC 50 value is calculated from determinations at
different concentrations. Activated sludge from a sewage treatment plant is normally used as
the microbial innoculum for the test.
Recommended tests and the identification of potential “HC”
The characteristics of “HC” are first, they elicit effects on aquatic organisms at very low
concentrations, and second, important pathways such as reproduction, development or cell
proliferation are affected. To obtain data that could give hints to such properties of a test
substance, the endpoints of the appropriate test should include reproductive success,
embryonic development and cell proliferation. In the Tier A assessment, tests determining
effects on reproduction are relevant in identifying “HC”. The Daphnia reproduction test
(OECD 211), gives information on the chronic potential of a test substance and its effect on
reproduction. Acute and subacute toxicity and developmental effects may be detected in the
fish early life stage test (OECD 210). During the Tier A assessment, the test on algae growth
(OECD 201) may point to inhibitory effects on cell proliferation. If a substance elicits growth
inhibition of algae, and affects the respiratory rate in activated sludge (OECD 209) at very
low concentrations, this would be detected in the standard tests.
Is the fish early life stage testing ELS sufficient enough for the identification of “HC”?
An extended early life stage test with zebrafish (McAllister and Kime, 2003) showed in case
of tributyltin (TBT) that effects on zebrafish in early development became apparent at
81
adulthood and were irreversible even after fish were held for 5 months in clean water.
Exposure up to 70 days post hatch to 0.1 ng/L TBT yielded a male biased population and
resulted in sperms lagging flagella. Exposure to 1 ng/L reduced sperm motility and exposure
to 10 ng/L resulted in a complete loss of sperm flagella. But the alterations in sperm motility
and assembly were examined 3 to 5 months after exposure to TBT. These results clearly
show that effects induced through substances can be missed by performing fish early life
stage tests only. In addition, this test last only a shorter period of time.
Bogers (Bogers et al., 2006) performed an extended fish early life stage test with fathead
minnow
exposed
to
methyldihydrotestosterone
(MDHT).
The
no
observed
effect
concentration (NOEC) for vitellogenin induction in males was above 1 µg/L MDTH for fish
exposed 63 days post hatch. This NOEC was reduced by a factor of more than 3 when the
fish were exposed for 114 days post hatch. For gonadal attachments the NOEC dropped
from above 1 µg/L MDTH to below 100 ng/L when exposed 114 days instead of 63 days post
hatch. This is another example showing that the fish early life stage test is not sufficient to
monitor all relevant effects of substances. In particular, adverse effects on fertility and
reproduction cannot be assessed by this short-term test.
In a prolonged ELS test with the marine flatfish sole (Solea solea), exposed to PCB 126
(pentachlorobiphenyl), Foekema (Foekema et al., 2008) showed that the fish early life stage
test may underestimate the toxic potential of compounds with low acute toxicity such as PCB
congeners. In a first experiment, eggs and larvae were exposed to PCB 126 until 15 days
post fertilisation (15 dpf), the moment that all fish had become free-feeding. At the moment
when all fish were free-feeding the LC50 ranged between 39 and 83 ng PCB 126/L. In a
second experiment, eggs and larvae were exposed until 4, 8, 10 and 15 days after
fertilisation (DPF) with PCB 126. Subsequently the development of the larvae was registered
under further unexposed conditions.). With this prolonged observation period, the LC50
ranged from 1.7-3.7 ng/L PCB 126 for the groups exposed for 4, 8 and 10 dpf, respectively.
This study indicates that ELS fish tests that are terminated shortly after the fish becomes
free-feeding, underestimate the toxic potential of compounds with low acute toxicity such as
PCBs.
Liao (Liao et al., 2008) showed that the sensitivity of fish to 17 beta-estradiol (E2) varies with
different life stages. Rare minnows were exposed to E2 at environmentally relevant (5-100
ng/L) and high (1000 ng/L) concentrations and induction of vitellogenin (VTG) and gonad
development were assessed. The LOEC for VTG induction in the adult stage was 25 ng/L
E2, but in the larval and juvenile stage the LOEC was 100 ng/L. This clearly demonstrated
that the VTG induction is more sensitive in the adult stage than in the larval/juvenile stage.
82
These data shows that the ELS test is not sufficient enough to monitor all effects of
pharmaceuticals/chemicals on fish.
Conclusions
This study indicates among others that the toxicity is not only dependent on the exposure
duration, but also on the life stage. The early life stage is not necessarily the most sensitive
life stage. Hence, chronic fish toxicity tests and tests on fish fertility and reproduction are
necessary for clear identification of “HC”.
Summary
The goal of the presented study was to develop a concept that enables the regulators to
identify potential highly active compounds “HC” with the help of existing data. We discussed
shortly the case of EE2 and gave a short description of the EMEA guideline.
To have a better overview about the existing pharmaceuticals, we made a catagorisation
according to the mode of action of existing pharmaceuticals.
During the next step, we search the literature and data bases for existing “HC”. We made a
summary of found pharmaceuticals that display effects at aquatic organisms at low
concentrations. Following this, we identified important pathways and receptors that could be
effected by “HC”. The main questions were: biological function of the pathway, evolutionary
conservation of the receptor, existing pharmaceuticals that display effects at these pathways
and ecotoxicological effects.
The main time, we spent for the identification respectively the development of concepts that
enables the regulators to identify potential “HC”. We developed the mode of action concept
with the main questions: what is the mode of action of a pharmaceutical, what is the degree
of homology between the human drug target and the potential drug target in non-human
species and how important for the organisme is the effected pathway. During the
development of this concept, we discussed also the usefulness of existing toxicological data
for the identification of “HC”. We made a detailed investigation of receptor aminosequence
homology between human and non-human species. And we applied the mode of action
concept using selected drugs to identify potential “HC”. In a next step, we described the fish
plasma model developed by Huggett and we applied this model for selected pharmaceutical.
83
We also investigated whether QSAR models would be useful for the identification of HC.
From the existing QSARs we described the VirtualTox lab in detail. We also used some
pharmaceuticals to apply this QSAR model.. At the end, we compared the outcome of all
three described concepts. The mode of action concept and the VirtualTox lab gave the same
results. The fish plasma model identified two compounds as “HC” which were not identified
by the other two models. But the reason for this, is the high environmental concentration of
both compounds.
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